An FPS on PSP? Doesn't sound like it'd work but...
The graphics are really crisp and detailed, looking better than some PS2 games (and dare I say, some xbox/gamecube games), but this is a Konami game, and they are graphics masters after all. [READ : Metal Gear Solid, Silent Hill, etc.] I didn't spot any graphical errors at all. This game is sexy on the PSP's sexy screen.
The gameplay is nice, fun, and even more so in multiplayer. A cool thing is that every playable level is generated randomly, which essentially adds endless replay value. The enemies are sometimes easy to kill, but the game's engine can handle many enemies on screen at once so the battles get pretty hectic. Unfortunately, gameplay can get repetitive and the game becomes boring at certain parts. Multiplayer fixes this though.
The sound is nice, with appropriate techno beats and whatnot. The guns sound really nice, as do everything else. Subtle footsteps, the enemy's battle-cries,etc. all sound wonderful. Overall, I suggest this game if you can find it. It may sound biased but this is a good title on PSP.