I'm kind of on the fence about Mega Evolution, as it gives some Pokemon like Absol and Mawile an evolution that they deserve (and Lucario a 3rd family member), but I don't see what would've been so bad with just making them regular evolutions. Then Pokemon like Ampharos and Blaziken who already are the third evolution in the family, go ahead an evolve once more, though they really didn't need it. And as for Mewtwo, I would've been perfectly fine with Mega Mewtwo being another form like so many other legendaries have now, rather than being only the second legendary to evolve.
So all in all, I've got to say I'm a little disappointed about these Mega Evolutions, but maybe they'll grow on me as we get closer to X/Y actually being released. And I hope that these aren't the only 6 Pokemon that get this special treatment, as there are quite a few others I can think of that probably deserved a Mega Evolution before those like Blaziken and Ampharos.