With great story and music, amazing online multiplayer and rewarding combat, Dark Mirror is one of the better PSP games.
The story is great, well thought out and mature; it's easy to see that a lot of time was spent on it unlike most of the other games on the PSP. However, the game's best parts are the music and the multiplayer. Mark Snow has outdone himself with the music; the soundtrack is superior to that of the rest of the series. The multiplayer is epic: you can play online with up to eight people in four areas, create cells, add buddies and earn badges. The better you play, the better weapons you get at the start of a mission. After a certain number of headshots, you get to wield more weapons.
The single player combat is also rewarding. You earn badges which unlock upgrades for stealth, headshots, surviving missions without dying once, and for using the environment against your enemies. The graphics, for a PSP game, are phenomenal.
The game makes great use of the buttons the PSP has: You control the camera with the face buttons, move with the analog stick, aim and fire with the triggers and do actions with the D-Pad.
The game is harder on the PSP for some reason than the PS2 version. Overall, the only noticeable problem is that Gabe keeps his weapon raised awkwardly, an annoying problem that was fixed in Logan's Shadow and Resistance: Retribution.
This game is definitely for you if you want a game with enjoyable online multiplayer.