The more I read about Mega Evolutions, the more I believe they were designed to be overpowered. One after another, they have got top-of-the-shelf abilities (except that one, you know), typing changes, and stat tweaks surely designed to break a hole or two in whatever opposes them. I mean, Kangaskhan attacking twice, and Attack-oriented Lucario with Adaptability? It's as if GameFreak themselves wanted more diversity in Smogon's Uber banlist.
That being said, although the Mega Evolutions are powerful on their own, it doesn't necessarily mean they will turn the tide of every battle. We still don't know their full summoning criteria. They can't hold items. They are limited to one per team, and I guess that there might be other things preventing them from making the rest of their team dead weight. Perhaps they will only work if their trainer are worse off in the battle than the opponent (for instance, having fewer useable Pokémon, or the Mega Pokémon being the last surviving team member, or another equally desperate situation). I think Mega Evolutions will act more like a last ace up the sleeve than a strategy viable on its own. A team relying on the Mega Evolution would have to be well-built to win, or they will be mowed down like grass before a scythe against a prepared opponent.