Well I guess I might as well share my thoughts on these new Mega Evolutions:
Mega Venusaur: Well like a lot of people have said before, I'm starting to see Mega Venusaur in a new light. At first I was like, "Where's the Mega Chlorophyll?", but now that I think about it, Thick Fat and a boost to its defenses will cause Venusaur to be one of the bulkiest Grass-types in the tier, and to tell you the truth, I'm fine with that. For starters, there isn't an abundant of bulky Grass-types in OU as of now. Breloom dies to boosted Hydro Pumps quite easily, so he's out of the equation. Ferrothorn is obviously great, but still being weak to Keldeo and being crippled so easily by burn blows, and Thundurus-T does a number to it with Focus Blast. Celebi is probably one of the best defensive Grass-types right now, but the huge risk of being trapped by that devilish T-Tar/Keldeo core + being weak to E-Belt HP Bug Keldeo is a shame. Amoonguss is an underrated bulky Grass-type, but it really has to watch out for Ice-type moves and Alakazam, which is being used quite a bit these days.
Now, with Mega Venusaur, OU finally has one of the best all around answers to offensive rain teams (if it's given 100+ defenses of course). Although it misses out on Leftovers recovery, it has the classic Leech Seed to help alleviate that problem. I think that making Venusaur bulkier in exchange for Leftovers is fine, because it has Leech Seed to alleviate that, and it's like giving Venusaur an Eviolite, which makes it bulky as fuck. If it's bulky as we are hoping, it will be one of the best counters to Keldeo in the tier, because it resists both of its STABs and takes a pitiful amount of damage from Icy Wind or HP Ice because of Thick Fat. Venusaur can then proceed to setup on it with Leech Seed or just straight up KO it with Giga Drain. Another important thing to note is that Venusaur doesn't have to worry about Toxic or Toxic Spikes because of its Poison-type, which will help out a lot of stall teams that need something to absorb T-Spikes and be a formidable wall. On top of that, Venusaur has a cool movepool as we all know, so it can run moves like Earthquake to get around Steel-types like Jirachi or Weather Ball if being used on sun teams to destroy Ferrothorn and Skarmory. Speaking of which, Venusaur will find a new found niche on sun stall teams. It resists almost everything that commonly fucks with sun, such as rain, and it can even do a number to specially defensive Heatran because of EQ if it has to. So all in all, I feel like Mega Venusaur is going be a great Mon. Yes standard Chlorophyll Venusaur will probably still get more usage, but Mega Venusaur will have an entire niche of its own that will allow it to function greatly in OU, and now then I think about it, if X and Y produces even more threats that can abuse rain, then Mega Venusaur will be that much more important. So who knows, maybe this will cause Venusaur to get a MUCH higher usage then it has now with just a sun sweeper niche.
Mega Charizard: When I first saw this, I flipped shit. Drought on a fucking Charizard? REALLY? That's INSANE!! Then I got to thinking that Stealth Rock is going to be a huge problem that might keep it from getting more usage then the shitty Ninetales, which is a shame. Sun is already crippled by SR enough as it is, but now its main weather inducer is 4x weak to it? Then I got to thinking, this thing is going to be a massive threat. Okay, first off it's getting a significant buff to its SpA stat as told by Serebii, so expect something like 125+, which is insane in the sun. Next off, Charizard is relatively fast, as 100 base speed is solid and it allows it to outspeed all the other weather inducers, while tieing with Ninetales. Charizard has a great movepool, with access to moves like Solarbeam to hit Politoed, Roost to help alleviate the Stealth Rock recoil, and Air Slash to hit anything that resists its STAB Fire move pretty damn hard. So sure, Charizard may be 4x weak to SR and it has to be given a bit more support to stay alive, but unlike Ninetales, Mega Charizard could pull its weight on the team by not being a complete liability and tearing massive holes in the process. So I highly doubt Charizard will be worse then Ninetales just because it's 4x weak to SR. It's just so much better in pretty much every way that I don't see a reason not to use it. Besides, ALL sun teams carry a spinner anyways. I'm never using Ninetales ever again unless I use both of them. That's all I'm saying.
Mega Blastoise: There honesly isn't much I can say about Mega Blastoise because they failed to give away any possible stat buffs, moves, etc. They were even very vague on its new ability, Mega Launcher, saying that it boosts pulse moves. I HIGHLY doubt that this ability is as shitty as it sounds, because I doubt Gamefreak would make an ability that boosts one move (Blastoise can't learn Dark Pulse or Dragon Pulse, just Water Pulse). I'm assuming it's boosting moves like Hydro Pump and Ice Beam that would be shot from its big cannon. Besides, every Mega Evolution has a great ability, so why would Mega Blastoise be given such a shitty ability? No clue. I'll talk more about this thing if anymore info gets released, but for now, I'm praying OU will have another viable spinner to use.