I don't like it. It strikes me very much like those ideas by little kids who think that a 'super-version' of a Pokemon is cool, so when I first heard of it, it seemed way too fanservicey for me. The designs aren't so good either - admittedly, some aren't so bad (like Mawile and Kangaskhan), but a lot of them seem to be either slapping new features onto the Pokemon or exaggerating old features with no sense of logical progression as to what making the Pokemon stronger would actually lead to. I mean... how does Heracross, a beetle-like creature, make you think of giving it air-vents of all things?
That, and Mega Evolution doesn't seem to fit with the Pokemon universe as a whole. A lot of the pre-existing Pokemon with forms had a reason for being that way - for example, Castform changes with the weather around it, Deoxys is a virus and so mutates a lot, Meloetta represents both forms of musical interaction. Mega Evolutions, by being slapped on to pre-existing Pokemon, have no thematic significance and so just feel like a super-powered form for the sake of a super-powered form. It's as close to a fourth evolution as they can get, really (I guess I should be glad that at least it isn't a fourth evolution...). The method seems off, as well, with the Mega Stones and Mega Rings. It isn't really clear why they exist in the context of the Pokemon world, so Mega Evolution feels even more out of place.
...And for all my griping, I think it was also poorly implemented. Literally only about 20 Pokemon out of 300+ evolutionary families get forms. There should be at least twice as many, in my opinion, to get a real feeling of balance. As it is now, it has a weird combination of focusing too much on the really popular Pokemon and having too many random Pokemon that feel like odd choices for this sort of thing. If there were more, it wouldn't feel like such a privilege to have one and so it wouldn't feel so weird. And Charizard should have definitely not had two, it's way too unfair to the other Kanto starters.
I know it's a lot of complaining, but really I just feel like Mega Evolutions are such a silly and sloppy concept.