[Vincent]

I... don't really get it either.
You just get a mega stone, and use a bracelet and behold! The pokemon mega evolves for that one fight. Then goes back to normal.
You can have them do it an infinite number of times. So I guess it's a way of choosing whether you wanna use the normal form or mega form for each battle.
Only BAD thing is that it takes away the 'hold item' slot. :c Meaning you can't give it an accessory to boost it's damage a bit or heal a bit more.
But then again you ARE giving it a mega-powerful form so. eh.

I like it. Mega forms sound and look cool and they introduce something new...
Kinda interested to see how it plays out in the anime, though...