Well, I have to say the two Mega evo's I am in love with at this point are Mega Aerodactyl and Mega Kangaskhan. My only sour thought on Mega Dactyl is how difficult it can be to make good use of his ability. To give him proper coverage, there are only two moves in the pool I've got for mine that actually 'make contact'. But, he makes a good anti Dragon team member with a higher damage Ice Fang.
Mega Khan is by far one of the more 'OP' I feel. I use her often enough, but the hitting twice and gaining effects of attacks twice is pretty nuts. Power Up punch, though weak, is a damaging Sword's Dance. Dizzy Punch becomes a pretty decent STAB attack with two chances to confuse your opponent. I feel her potential is really untapped at this point. I think she will continue to be my most used Mega at this point.
As for the mechanic itself, I feel its...interesting, to say the least. I know loads of people have compared the concept to 'Digimon' with their 'Divigolution' (Evolution that lasts only for brief periods of time, though enchances stats drastically, and provides stronger attacks). I won't be so vain as to say I was one of those people at first. But when I fist turned Lucario Mega during the storyline, I pretty much felt a childlike glee. The sound effects and visuals when it happens look and feel so good. Their new cry giving me tingles of excitement.
I honestly don't know how it will change the game overall. Its sort of a 'last ditch effort' kind of idea, where you forgoe the usual battle item in order to gain a boost of power which may, or may not, ruin your team (depending on type changes, if applicable). I do like its restricted to one Mega evo per battle. And I really love how it doesn't wear off if you switch out, so you don't basically 'waste' it if you have to change out for type advantage.
Though, I can't help a part of me being a bit disapointed. A lot of the Mega Evos could have been a true evolution to certain Pokémon. I know its a very common argument, but I can't help how I feel about it. Absol, Mawile, basically the basic stage Pokémon, and even Medicham maybe should have gotten full evolutions. I could see temporary evolution for stage 2 Pokémon, since I don't foresee them introducing a stage 3 any time soon.
But, those be my thoughts!