Here's a genuine opinion on all the new megas so far.
Altaria: We finally get a Dragon/Fairy type, which is something I would assume we have all wanted for quite a while. His fantastic typing which gives him only three weakness and a ton of resistances makes some people think that he's going to play a defensive role, but I think quite the opposite. You think is he were to be defensive, he would keep Natural Cure or gain a new ability that complements being a tanky as possible. I think he's going to play a more offensive role. Dragon and Fairy gives really great type coverage, except against steel-types. However, he does get a few fire moves, which should help out. The design is alright, if you just look at just the head he looks like some sort of old wizard, which makes me think he's primarily going to get a special attack buff.
Lopunny: Scrappy will make her a lot more viable as she now gains the ability to counter a lot of ghosts in the game. I'd assume that she also gains a lot of attack and speed, so much so to make her a lot more useful in higher tiers. This will also add a very unique playstyle aside from what she normally runs nowadays. Is it a physical set or one of those gimmicky item passing sets? Players asking questions like this then sequential losing because they guessed wrong is the reason some megas jumped up a tier or two. I never liked her design before but this makes her look like some sort of women's boxing champion, which is pretty neat. I also just noticed that this is a new mega-evolution of a gen 4 Pokemon. I had this feeling that these games were going to do what the gen 2 remakes did and not change what you encountered, pretty much locking off half the available Pokemon until you reach postgame. The fact that they are advertising a gen 4 mega that you will most likely be able to obtain makes me think that they are going to chance that, which I really like.
Salamence: I'm going to be blunt, I don't like his design. He ditches his arms for goofy wings that make me want to throw him like a boomerang. Salamence can go mixed as of now however I think that's going to change when we finally get to play around with this guy. This is because I think the wings look like blades, which in turn will probably have him net a big attack boost when he mega evolves. He has a pretty good movepool both physically and specially, but this mega will further enforce his power physical arsenal. I am however worried that this might make him too good. If any mega is going to be banned to ubers next it's probably going to be this. I mean, it's Salamence!