At first, I didn't really like the idea of Mega evolutions, because they felt a lot like easy to obtain pseudo legendaries. I never really cared to use them outside of cmpetitive battling, because of their raw power colliding with my playstyle. When it comes to competitive battling, what disturbs me, is that stupid matchup dependency it brings. There's so many megas to prepare for, that there's a high chance, you'll run into an opponent, who uses exactly that mega, you couldn't prepare for, or even if you prepare for it, you had to prepare for so many megas, that you couldn't prepare for powerful non-megas.
I do admit, though, while using them, I started liking a couple megas a little bit more, like Mega Scizor.