Patapon 2 is the best reason to have a PSP. It's a 2D strategy-action game in which you command a small, customisable army of creatures called Patapons, who come in a myriad shapes and sizes, and who must fight off dragons, giant spiders, and woolly mammoths in order to reach Earths End. It is genuinely eccentric and completely brilliant.
First, negatives: you have to be willing to replay missions over and over in order to progress. The game is designed this way - a level 1 dragon is just the beginning, you have to get to level 5 or 6 for the collectables you really need to build your army, and level 20 and beyond until he drops the crazy sword of death that you've been hunting. However, this is mitigated by the fact that you are effectively testing out your new, stronger army with each new battle, and experimenting with different units in order to find out the best strategy to win quick and clean. Some players found it a grind - I didn't.
Personally, I didn't have any other problems with it.
Positives: It has a highly individual and charming graphic style which is always fun to watch, hysterical sound design, and simple, addictive, rewarding gameplay. The learning curve is steeper than average, but once you get it, there's no going back. I put about 40 hours into it to finish the game and level up a (fairly) serious Patapon army, and I'm still going back to it. In short, it's keeping me away from some serious AAA PS3 and X360 titles that are all gathering dust. Ah well.