I'm going to start off by saying I'm going to look past my bias on what Pokemon the Mega Evolution was and instead treat the Mega Evolution as its own Pokemon. I may disagree with the idea of them trying to change stats around to make a Pokemon into something its normal form wasn't, like a Physical attacker to a Special attacker, but what's done has been done so there's no point in arguing about it anymore. Also, I'm only going to give quick comments, nothing inidepth as I pretty much said all I wanted about them when they were first revealed to us. So with that all said, let's beign:
Swampert:
Type: Water/Ground
Ability: Swift Swim
Norm: 100/110/90/85/90/60//535
Mega: 100/150/110/95/110/70//635
Change: 0/+40/+20/+10/+20/+10//+100
Opinion: Wow, Mega Swampert is essentially a brick wall that has a hell of punch! Its Waterfall and Earthquake is now devastating along with coverage from Hammer Arm, Rock Slide/Stone Edge, and Brick Break/Power-Up Punch. Speaking of punches, wonder if it'll get the elemental punched? It could really use Fire Punch of Ice Punch to deal with Grass-types (even with its higher defense its still quadruple weak to them). And of course you'll want someone who can set up rain support so that it goes from a hard-hitting brick wall to a speedy unstoppable force. Pretty much Mega Swampert is the juggernaut, bitch.
Sceptile:
Type: Grass/Dragon
Ability: Lightning Rod
Norm: 70/85/65/105/85/120//530
Mega: 70/110/75/145/85/145//630
Change: 0/+25/+10/+40/0/+25//+100
Opinion: Hard hitting brick wall not your think? How about a semi-fragile special sweeping speedster? With a Speed of 145 tied with being the 7th fastest Pokemon in the game. Only Deoxys-S, Ninjask, Deoxys-N, Deoxys-A, M-Alakazam, & M-Aerodactyl beats it while its tied with Accelgor. What didn't go into Speed went into its offense, mainly Special Attack. Leaf Storm from this thing is going to hurt. Of course I'd imagine people would rather have Giga Drain cause it'll heal itself a LOT with it. Its Attack is even decent that you could also give is Leaf Blade for those Special Walls. And of course we can't forget about how hard it'll be hitting with its Dargon-type attacks. And of course if you play smart you might even be able to switch it into an Electric-type move and increases it crazy high Special Attack further. Though hopefully they'll expand upon its Special movepool, it doesn't really have that many to choose from.
Sableye:
Type: Dark/Ghost
Ability: Magic Bounce
Norm: 50/75/75/65/65/50//380
Mega: 50/85/125/85/115/20//480
Change: 0/+10/+50/+20/+50/-30//+100
Opinion: Oh boy, you can tell there may have been a bit of a struggle here. Now its defenses aren't that bad, it could make for a decent wall and its type gives it some nice immunities. It also has Magic Bounce to at least stall your opponent's attempt to hinder you, but I think that's all Mega Sableye will be able to do, stall (which ironically is one of its normal Abilities). Though it'll become a speed demon in a Trick Room team and already relied heavily on Shadow Sneak, it's really kind of hard to see it getting much use. There are just other better defensive Pokemon and Sableye just doesn't have the power it needs to go along with its large defense. I mean I sort of get what they were trying here, it's counterpart is Mawile who they made into an offensive threat so they thought they'd make Sableye a complementary wall. However I don't think Sableye was made with it being a wall in mind (it has 50 HP). Maybe in Doubles it can get some use to help its partner set up, but on its own, unless it can completely wall its opponent or get lucky with using Confuse Ray and Protect/Detect, I don't think it'll do anything but stall for a bit.
Altaria:
Type: Dragon/Fairy
Ability: Pixilate
Norm: 75/70/90/70/105/80//490
Mega: 75/110/110/110/105/80//590
Changes: 0/+40/+20/+40/0/0//+100
Opinion: Huh, I guess that's really all I have to say. I mean its okay, its offense and defense stats are all above 100 so you can sort of play with how you can use it. It can be a mixed attacker, a tank, physical focus, special focus, it's moldable which is to its advantage as your opponent won't know quite to expect. It's typing further helps with any set you go with, though its Ability is urging you to play a bit more offensive. That said, it doesn't exactly excite. It's balanced to the point I'd say its too balanced for its own good. I'll end it by saying, like its stats, while it doesn't fail it doesn't quite excel either.
Gallade:
Type: Psychic/Fighting
Ability: Inner Focus
Norm: 68/125/65/65/115/80//518
Mega: 68/165/95/65/115/110//618
Changes: 0/+40/+30/0/0/+30//+100
Opinion: Now here we go! Mega Gallade pretty much got the boosts in the stats he needed, Attack and Speed! Infact it's gotten so fast it no outpaces Mega Gardevoir which it sort of needed to do. Okay, Inner Focus isn't the best Ability, but its Attack is high enough that it doesn't matter it hits even harder with STAB Close Combat and Psycho Cut with coverage from its other handful of Physical moves (it may want to think packing Poison Jab for Faerie-types)!
Audino:
Type: Normal/Fairy
Ability: Healer
Norm: 103/60/86/60/86/50//445
Mega: 103/60/126/80/126/50//545
Changes: 0/0/+40/+20/+40/0//+100
Opinion: Well, it took no time to find a better defensive Mega Pokemon then Mega Sableye, I wasn't even looking for one and Mega Audino just walk right up to me! While it may not have Magic Bounce or that many immunities, its defenses aren't only higher but it has the high HP it needs to wall hits. And with Healer and learning Heal Pulse, it does carve itself a niche in Doubles more deeply then Mega Sableye. Though on its own its sort of on the same boat as Mega Sableye. That said, if you put the two up against each other Mega Audino has the advantage as it's higher Special Attack and new Fairy-typing means it'll probably be packing Gazzling Gleam, and Sableye is weak to Fairy-type attacks.
Sharpedo:
Type: Water/Dark
Ability: Strong Jaw
Norm: 70/120/40/95/40/95//460
Mega: 70/140/70/110/65/105//560
Changes: 0/+20/+30/+15/+25/+10//+100
Opinion: Would have liked a higher Attack but hey, 140 is still very nice, they made it a bit more durable, and it's Speed is above 100, what more can you ask? Even its Special Attack is still usable incase you encounter a Physical wall. Of course the main focus with Sharpedo is its high Attack and giving it as many biting moves as it can have. Crunch and Ice Fang are give-ins and I even hear it now gets Poison Fang too! Oh, and of course it needs a physical Water-type so you can decide between higher priority Aqua Jet or stronger hitting Waterfall.
Slowbro:
Type: Water/Psychic
Ability: Shell Armor
Norm: 95/75/110/100/80/30//490
Mega: 95/75/180/130/80/30//590
Changes: 0/0/+70/+30/0/0//+100
Opinion: Oh my. I'll be honest, I didn't know what to expect from Mega Slowbro, and apparently the designers equally didn't know what to do with it. So they just boosted its two best stats and called it a day. And you know what, I'd say it sort of works. Yeah, Mega Slowbro is a pretty viable physical wall now with okay Special Defense and can deliver a mighty powerful Special attack to boot! It'll be attacking last of course, but that's no different from normal Slowbro (and once again, Trick Room is a thing). Give it Slack Off for healing, Surf and Psychic for attacks, and a fourth move that meets your fancy and Mega Slowbro looks good to go! And you don't even need to worry about crit hax.
Steelix:
Type: Steel/Ground
Ability: Sand Force
Norm: 75/85/200/55/65/30//510
Mega: 75/125/230/55/95/30//610
Changes: 0/+40/+30/0/+30/0//+100
Opinion: Mega Slowbro except a physical attacker instead of special. Well, statwise, of course its Type and Ability sort of has it play differently. Mega Steelix wants you to use it on a Sandstorm team to boost its STABs (and damage your opponent every turn while it doesn't get scratched) like Earthquake and Gyro Ball, not to mention its other physical coverage. While Slowbro is a bit passive on how it attacks you, Steelix is a bit more straight-forward about it.
And quickly to talk about its design, I find it looks cool but a bit jarring. I like how it's spikes have finally turned into diamonds after its Dex entry says it was as hard as diamonds, and also its head turning into a shovel sort of goes along with its serpent body. However those sections of its body with the blue octagons and it looking to gave gained a magnetic field making pieces of ore float around it kind of comes out of nowhere. It's still definitely Mega Steelix, though it sort of looks like they had two design ideas (on to focus on it being hard as diamonds & a digging Pokemon while the other wanted to focus on it being metallic & make it magnetic) and decided to combine them together.
Pidgeot:
Type: Normal/Flying
Ability: No Guard
Norm: 83/80/75/70/70/101//479
Mega: 83/80/80/135/80/121//579
Changes: 0/0/+5/+65/+10/+20//+100
Opinion: ... *sigh* Fine, I can admit when I'm wrong. Mega Pidgeot is a pretty awesome Special Attacker, but of course they did do their best to make it work (which makes me wonder what happened with Sableye and Altaria). It has a pretty high Special Attack and great Speed to essentially make sure it'll be hitting you with 100% Hurricanes first. It's a speedy Special sweeper and that's all it needed to be to prove it can work!
Glalie:
Type: Ice
Ability: Refrigerate
Norm: 80/80/80/80/80/80//480
Mega: 80/120/80/120/80/100//580
Changes: 0/+40/0/+40/0/+20//+100
Opinion: It's been mentioned a dozen times but I'll mention it again: Ice-type Explosion. Actually it makes me wish they put more in Attack, but what we got is still good. Actually, like with Mega Slowbro, I don't think they knew what to do with it so they increased its offense, gave it an even 100 Speed, and then gave it to us and told us to do something with it. Well it has a few things that we can work with, like Ice-type Headbutt/Return plus a nice amount of Special moves it can work with, but in the end everyone knows what it'll end up doing. Ice-type Explosion. Because Sheer Cold isn't allowed and has a high chance of missing.
Oh, and quickly talking about its design: What is this I don't even. Okay, I get the ice horns and sort of the gaping maw, but what's with the heavy beard? I mean I know there's not much to Glalie's desing to work off from, but even the a beard is just random. Maybe when they put it on the site they'll explain it, but for now it looks off with it.
Diance:
Type: Rock/Fairy
Ability: Magic Bounce
Norm: 50/100/150/100/150/50//600
Mega: 50/160/110/160/110/110//700
Changes: 0/+60/-40/+60/-40/+60//+100
Opinion: ... AND the award to the Pokemon who's stats changes the most upon Mega Evolving goes to... DIANCIE! I mean, WOW, it goes from being an okay defensive Pokemon to a powerhouse in a snap! It's stats goes through a total of 260 point changes! However they made it work and now a Pokemon you were just treating as a Carbink that could hit harder becomes a Pokemon who will pulverize you with its Signature Rock-type move or just a normally strong Rock- or Fairy-type move! And of course it even still makes for a good defensive Pokemon as it gets Magic Bounce (*Mega Sableye throws down its jewel in anger and storms off*) to help stop your opponent from hindering your team. Of course it'll probably be more focuses trying to knock Mega Diancie out, it still only has 50 HP.
I would comment on Mega Diancie's design but how pink it is is making my Y chromosome trying to make me look at something more manly. I'll just say that she's a very pretty, pretty princess... you can annihilate you if she feels like it.
Metagross:
Type: Steel/Psychic
Ability: Tough Claws
Norm: 80/135/130/95/90/70//600
Mega: 80/145/150/105/110/110//700
Changes: 0/+10/+20/+10/+20/+40//+100
Opinion: Steve might as well turn to you and say "GG man, GG". So yeah, Metagross is now speedy which means one of your main advantages over it is now gone as it punches whole in your team with its four fists. Actually I'm surprised it doesn't get a higher Attack boost, but with Tough Claws does it really need it? Use those points to make yourself harder to knock out and able to outspeed most Pokemon, that'll make Steven a possible nightmare to deal with. At least now its weak to Dark- and Ghost-type attacks.
Kyogre:
Type: Water
Ability: Primordial Sea
Norm: 100/100/90/150/140/90//670
Primal: 100/150/90/180/160/90//770
Changes: 0/+50/0/+30/+20/0//+100
Opinion: ... What? +50 to attack? Who would ever use a Physical Kyogre? Like, okay, I suppose it can now use a Physical Move if it wanted, but with a Special Attack of 180 I think people are going to focus on that. A bit of waste of points there, but some of that in Speed at least! Eitherway, Kyogre is already good, its in Ubers, so what did you think its Primal form was going to do? It's not as tier breaking as we thought it would be, but it'll still make most Pokemon head for higher land if they see it coming.
Groudon:
Type: Ground/Fire
Ability: Desolate Land
Norm: 100/150/140/100/90/90//670
Primal: 100/180/160/150/90/90//770
Changes: 0/+30/+20/+50/+0/0//+100
Opinion: Pretty much same thoughts as Kyogre except with it getting the increase in Special Attack. Actually there is more to say about Groudon as its Ability makes it immune to one of its major weaknesses thus sort of making it the top dog around here (well, except for the Pokemon up next). And I suppose that Special Attack increase helps give it some usable Fire-type moves, unless it gets Flare Blitz of course (or Sacred Fire. You know what, let' just give it V-Create and end it all here!).
Rayquaza:
Type: Dragon/Flying
Ability: Delta Stream
Norm: 105/150/90/150/90/95//680
Mega: 105/180/100/180/100/115//780
Changes: 0/+30/+10/+30/+10/+20//+100
Opinion: Ah, there we go, NOW we can break the Uber tier in half and anguish as Mega Rayquaza laughs as it burns it. And with only only normally weak to Dragon, Ice, and Fairy-type moves and an increased Speed stat above 100 it'll probably do it for awhile. It now has a Signature Move that'll hit Fairy-types for heavy damage and for everything else, Outrage (okay, maybe a Fire-type attack for Steel-types... do you think they'll ever release V-Create Rayquaza again?).
Camerupt:
Type: Fire/Ground
Ability: Sheer Force
Norm: 70/100/70/105/75/40//460
Mega: 70/120/100/145/105/20//560
Changes: 0/+20/+30/+40/+30/-20//+100
Opinion: Decrease in Speed? Didn't expect that, then again it wasn't fast anyway and now has a volcano on its back (and just means it'll likely go first on a Trick Room team). So yeah, with it getting Sheer Force it mainly needed to focus on its offensive stats which it did. It got a nice buff to its defensive stats too but sadly I don't think it'll help it much (unless its in Doubles with Primal Groudon, but I had this odd feeling Primal Groudon would probably just eat Mega Camerupt for subsistence instead of battling with it). But yeah, Trick Room + Fire-type attacks and Earth Power/Earthquake could help Mega Camerupt go far. If also on a Sun Team it may want Solar Beam too for Water-types.
Lopunny:
Type: Normal/Fighting
Ability: Scrappy
Norm: 65/76/84/54/96/105//480
Mega: 65/136/94/54/96/135//580
Changes: 0/+60/+10/0/0/+30//+100
Opinion: Forget how weird Lopunny stats were. Anyway this thing is kicking all sorts of @$$. Like Gallade it needed an increase to Attack and Speed and it got it, though it gives up power for more Speed. Just give it Hi Jump Kick and let it fly!
Salamence:
Type: Dragon/Flying
Ability: Aerilate
Norm: 95/135/80/110/80/100//600
Mega: 95/145/130/120/90/120//700
Changes: 0/+10/+50/+10/+10/+20//+100
Opinion: Oh yeah, forgot Gen III had a second Pseudo-Legendary. Actually Salamence sort of follows a similarly to what Metagross did. Gave it's Attack only a slight increase, focuses on Defense and Speed, and gave a few points to everything else. And though Salamence doesn't have Tough Claws, it does have Aerilate to give it STAB Flying-type Returns (which Aerilate also increases), not to mention it also having Double-Edge and Thrash. Plus its Special Attack is good enough to use Draco Meteor one or twice with before focusing going Physical (a strategy you can also do with Mega Rayquaza since its new Signature Move is also Physical!).
Beedrill:
Type: Bug/Poison
Ability: Adaptability
Norm: 65/90/40/45/80/75//395
Mega: 65/150/40/15/80/145//495
Changes: 0/+60/0/-30/0/+70//+100
Opinion: LOL! I like what the designers did here. F@#$ defense, drain Special Attack because it's not using it, and shove as many points in Attack and Speed as you can. Mega Beedrill may not last long in battle but when it is it'll definitely do its best to leave a lasting impression! Especially if it can get Fell Stinger to work...