LocoRoco is probably the cutest thing that you'll ever encounter for the PSP.
You tilt and bounce your little blob through a colorful world growing in while numbers (and size) along the way. While there are a few enemies that attempt to hinder your path, they never really present too much of a threat. The real challenge comes from trying to hunt down every single secret passage and the quest to find all of the hidden mini-game pieces.
If you are merely interested in beating the game, you can probably do it in about 5 hours. However, if you want to get a "perfect" on every level and unlock all of the hidden content (mini-game stuff, music tracks) it is probably going to take you a bit more time.
LocoRoco is remarkably straight-forward in design and controls, never deviating too much from the path it lays out in the first level. That said, it is really pretty hard to put it down. It is probably not going to be loved by everyone, but if you own a PSP, give it a shot.