[Question] What's a good single-player FPS game?

Deus Ex definitely takes some getting used to, though. It wasn't for me, but I hope Bioshock will appeal a bit more.

Aliens vs. Predator 2 has an excellent single player campaign, because it's actually three campaigns in one. You can play the marine version (obviously the easiest to identify with, and probably the best), the predator version, or the alien version. All three classes in the game have different aspects about them that make them pretty interesting to play. I can't stress enough the basic fun factor of having your pants scared off of you. (or of dropping stealthily down on top of a marine and headbiting him)

Plus, when you get done, the multiplayer is a real blast, although I imagine you'd have trouble finding a server nowadays.

FarCry has some great single player action, especially in the human vs. human battling. You really feel like you outwit these guys sometimes, rather than feeling like you've somehow cheated the AI. Make sure to turn on dynamic difficulty, though, or your trip can be a painful and frustrating one, especially in the last few levels of the game.

There's also Call of Duty 1 & 2, if you haven't gotten around to playing them yet. Call of Duty 2 can also run in Direct X 7 mode, which is a real system saver.

For a more tactical/more serious approach, you could go the route of Rainbow Six 3, or Swat 4, which are similar, but both good in their own rights.

Jam it back in, in the dark.