Can someone describe to me what the Mega Evolutions did for the competitive endgame? Besides not being able to use another item, it just seems like there are no downsides, which is stupid. It doesn't even cost a turn to transform.
Coming from someone who plays Ubers and Battle Spot, the cool thing Mega Evolution brings is to team building. Teams can not have more than one Mega Evolution per battle, which is very interesting team building wise. For example, in Ubers I sometimes run an offensive team based around Wobbuffet setting up dangerous sweepers like Xerneas and Blaziken. In this kind of team, I have had 2 very serious choices for Mega Evolutions: I could use Mega Blaziken so I can use Adamant (the extra Speed means I can outspeed stuff like Mewtwo without Jolly), however, I can also opt for Mega Gengar to help trap stuff that Wobbuffet can't trap as easily (plus Wobbuffet and Mega Gengar are very nasty piece of work together), however, this comes at the cost of having to use Jolly Life Orb Blaziken. Or hell, I can use other Mega Evolutions as well, but the big point here is that I can't have them all at once, which is a pretty big shake up team building wise!
As for Battle Spot metagames, because you don't send out your entire team (unless its Triples), you can actually get away with MULTIPLE Mega evolutions, but having them both out at once usually isn't a good idea against most teams, so it adds a sorta of risk to team building: Do I use two Mega Evolutions to help against certain match ups, or does using two Mega Evolutions limit my team's versatility too much?
This can get even crazier in doubles. For example, I had a VGC team that had both Mega Manectric and Mega Charizard Y. Occasionally, I'd surprise some people by leading with Mega Charizard-Y, and then switching in Manectric into a predicted Electric move to get a Lightning Rod boost, and saving Mega Charizard Y from a big hit!
Overall, while Mega Evolution powers up most Pokémon, it also adds a whole new layer to team building because of its restrictions!
EDIT: I also forgot to mention: Mega Evolution does not affect your Speed stat until the next turn. This can be pretty annoying when stuff like Mega Lucario can't outspeed say, Palkia on turn 1.