Can anyone tell me what the negative side of Mega Evolutions is? I'm playing Y right now (review copy) and I can't really find a reason not to Mega Evolve at the start of battle.
There are a few situations where you'll be able to Mega Evolve but not want to press the button, but most of them won't happen often:
-If it's a mega evolution that changes type and it'd cause you to be at a type disadvantage.
-If it's something like Charizard where the weather's already up but your opponent has the ability to change it so you want to wait until you need to change the weather again, unless mega evolving gets you an important KO.
-Against human opponents, when using, say, Garchomp, you might want to try to trick your opponents into thinking you have a different item. This will only be practical for Pokemon that are good w/o mega evolving, though.
Basically, the reason you can time it is for those tricky situations and not because there's a hard drawback.