I'm a huge fan of the whole MegaEvolutions concept. It'll open up a brand new way for us to do battles, and it will give us the chance to see the competitive battling scene through a different perspective. It may take us a while to adapt to this system, but we'll get used to it eventually.
My favorite MegaEvolution so far would have to be Mega Mewtwo X. It's got a pretty fierce design, and the way that it looks at you is simply awesome! I also like Mega Charizard Y. It's a lot slimmer than its original form, so I'm guessing it'll be able to move quite fast and dodge a few hits. d: Mega Charizard X doesn't look too bad, either, but I'm just not too fond of its new body color. Black and blue just don't go along with it very well. If it kept its original color scheme—orange and khaki—then it would have looked ten-times better, I'm sure. Mega Charizard X will be quite powerful on the offensive side of things, since it's getting a major increase in its Attack stat.
Since Charizard is getting two Mega forms, I think they should give a second MegaEvolution to Blastoise and Venusaur as well. Charizard, Venusaur, and Blastoise are all equal, so they should be treated as such.