One of the best PSP games out there.

Syphon Filter: Dark Mirrior is quite an impressive game, the single-player is a little difficult, but once you get the feel for it, you'll have a fun time. The graphics are pretty good considering this is a PSP game, but Gabe looks more detailed than the other models. I wont talk about the SP much, because this is the best Multiplayer PSP game out right now.
At first the MP is frustrating, because the higher ranked a player is, the more weapons he gets, so you get junk.
However, once you make it to rank O1 (which takes a long time) things get more evened out. The MP is intense, even though there can only be 8 people, the battles are frenzied. There are many ways to kill someone, push 'em of the ledge, break their neck, zap them, dart them (only in objective mode though) shoot 'em, or my favorite, break their neck (which is extremely hard to do in MP, but it's very satisfying)
Overall, this is a great game if you're a fan of the stealth/action genre, with a challenging and great single-player, and an addictive multi-player.