Fun, unlimited lifetime

I say more than what is in the title. Basic play is ultra fun, the basics of maps are. But in addition, the game has a built-in level creation tool and a community that produces pure wonders sometimes better than the basic maps.
And add to that the fact you can play cooperatively (well, this is relative because it will always end by slapping in-game battle) and we absolutely have to have a game for any PS3 owner.