Double Battles are fun, if there is nobody that plays Skuntank G/Crobat Prime (I was lol with Skuntank G). Basically, the one before me is going to die even with a Wailord on active (once, he actually had it), just with noob attacks and Poison. Then, I just try to KO the other one. It was pretty cheap.
It is fun if you play with good friends, because you laugh a lot. If you just find 3 other random guys, it will be boring and long (very long).
Oh, and for us, when a team gets a KO on an opponent's Pokemon, both players takes a prize. Therefore, you don't need to take 12 to win the game.