Honestly, while I'm still not really a fan, I think my main problems are, 1) the designs, and 2) the fact they're battle-only.
The designs, especially Lucario, Blaziken, Ampharos, and Absol, are really just "add stuff to make it look cooler!" If they looked a little more different (a la MegaMewtwo) or like actual evolutions (like MegaMawile), I'd be a little more accepting.
This brings me to my next point...the fact they're "battle-only." I'm very likely in the minority, but because this is why they remind me of Digimon (well, that and an extra "evolution"). If they bit the bullet and made Mega Evolution true evolutions (and not temporary form changes), I'd be more shocked initially but more accepting in the long run. It would feel less like Digimon and more "woah what the...?!"
That being said, given time to digest, I'm still not thrilled, but I am slightly less hesitant.