Short-lasting eye candy.
The missions are quite nice, but there are only 4 areas where you can play, and the last one consists solely of a single boss. The first 3 areas are laughably easy, once you realize where to go and what to do, you can beat any mission. Probably to make up for that, the final boss is very hard - without very good timing and reflexes, and a large bit of luck, you have no chance.
In total, the game offers 4, maybe 5 hours of gameplay, and then you're done. No unlockables, no secrets, nada. There are some downloadable maps on yourpsp.com, but they seem to be multiplayer-only.
So, DO NOT get this game, unless you know a few PSP-equipped folks who'll get it too - multiplayer is rather nice, but the single player mode is a complete rip-off.