Split screen can be fun or a nuisance depending on the game.
Things like Mario Kart is hilarious when your friends throw the controller at the TV for getting hit by your blue shell. Happy days...
But in the more FPS games (think COD, Halo) I find it a hassle with screen cheating. A prime example of this is when I'm trying to head shot someone with the sniper, they glance at my screen and hide to a spot where I can't see them.