Essentially, Mega Evolutions are the temporary in-battle power-ups. They only bring about new Abilities and, for some, Typing. You keep the same moveset, so part of it is planning ahead on what roll you want for your Mega Evolved Pokemon since different stats are affected for each one. There are limitations in how many Pokemon you can Mega Evolve during the course of a battle. I believe it's only 1.

I do love the variety as well. It keeps so many options for a team wide open.

From what it looks like, a majority of the collection process begins after the whole Elite Four. I'm curious if the whole of Kalos is open before or after the Elite Four like Black/White's region was.