Yeah, PSP is totally cheap right now to pick up games for, I've been scoping out Gamestops near me pretty frequently to grab the used stuff that comes in. Thankfully they don't gut the games either, so the ones that come in complete are sold that way.
If you like puzzle games, Crush is a very fun and addictive game that not a lot of folks know about. I also recommend Half Minute Hero as well, and there's even a localized Dungeon Explorer game by Hudson for the PSP!