I think this game's A.I. is still
impressive today - especially for a game that came out in 2004. Half-Life 2 seems to get all the attention (it should get attention, it's a great game), but, to me, F.E.A.R was more engaging and fun to play, with vastly superior A.I. HL2 is artistically / graphically a little better, but the gun-play, player movement (you don't feel like you're floating in "dev mode" like you do in HL2), ability to lean, multiple melee moves, and the surprisingly un-gimmicky "reflex" mode IMO make it the better FPS of the two.
Buy them both, though - - they're cheap.