I'm actually surprised a majority of votes are more positive. As for the people viewing it negatively, more likely than not it will be strictly optional. I have my doubts any NPC trainers, except maybe the last few Gyms, Elites and Champion will have access to MegaVolution. I'm also questioning if it will carry over to battles between trainers, but perhaps there will be a setting when preparing/selecting a match to leave MegaVolutions off. In short, we don't know much about MegaVolution yet, so don't let it ruin your opinion of the game and stop you from getting it.
I love them and am hoping that it is activated by holding that pokemons "mega stone" (i.e. the Blazekinite) that way you will have to choose between mega forme or say a choice band focus scarf leftovers etc. Blaziken will lose its scarf damage but will be stronger so about equal there. Mawile gains Huge Power and higher attack than its already impressive base attack reaching 595 attack with adamant nature, so its a clear choice there. Absol I will have to see the updated stats to say whether to use mega or an item. Ampharos i say mega purely for the dragon typing. Lucario i say mega. Mewtwo i say use something else hes already good enough.