I would avoid the sequel, though, which is called "Ties That Bind". For some reason I can't explain, it just wasn't as fun as the original.
Upside: Awesomely detailed and realistic weapons, excellent music, fairly intelligent AI, some of the best graphics on PS2, and of course, very destructible enviroments. The explosions in this game are really something to behold. Oh, and the mission areas are truly gigantic, which gives it a cinematic feel.
It doesn't really do anything new for the genre (but then again, I don't think it was meant to), but it executes the old formula pretty well.
Timesplitters and Red Faction (I'd say get the second or third Timesplitters and the first Red Faction) are the best if you ask me. As JLH said Ratchet and Clank 2 and 3 aren't FPS but they are the best shooters in general on the system, as well as being among the best games period on the PS2.