Such a great game. Wacky, casual, fun... If you've got a PS2 and you haven't played it yet, you should. It's coming to the PSP soon as well (though I've heard complaints about the controls...). Check out the Gamespot review here, with pics and video. (Gamestop, by the way, is officially my favorite game site. It's just got a lot better quality content than GameSpy or IGN).
Besides just being a complete acid trip, Katamari is very innovative in that the gameplay is simplistic, yet complex. You have this ball which rolls up the things around you - but you start small and can only roll up things of a relative size. Later, as the ball gets bigger and bigger, you can pick up bigger items. The cool thing is that the things which were obstacles early in a round become targets later on. And it's amazing the things you end up picking up later. The reason everything is so blocky is to save polygons because *everything* it seems in your environment is able to be picked up eventually - from the smallest items like matchsticks to people, cars, buildings and more.
Definitely one to pick up - I can't wait for the PSP version.