When Mega Evolutions first came out, I was very much opposed to the idea. I thought it was going to throw off the balance of power in the game and in some ways I really feel it did. If you're battling someone who has a mega on their team, but you don't, it puts you at a disadvantage.
I also feel as though Mega Evolution has been geared towards making the strong, stronger, instead of making Pokemon on the cusp of being competitive or need help to be in a stronger position viable. Like don't get me wrong, some Pokemon like M-Pidgeot, M-Sableye, M-Audino, and M-Lopunny might be used more because of their Megas, but I feel like far more already powerful Pokemon received Megas that, quite frankly, they didn't even really need to be competitive and used.
All of that being said, however, I feel like Megas have grown on me a bit since they came out in XY simply because it's just part of the game now. There's really not much I can do besides accept it and try to use it to my benefit, so that's what I do and try to accomplish. No matter what anyone thinks of Mega Evolution, they're here to stay and we might as well make the most of it. I just hope they decide to make more Megas that will allow a larger range of Pokemon to be used.
(I also accept it because shiny M-Gardevoir looks amazing :P )