My favorites/future buys (all shooters):
[in no particular order]
Bioshock
Gears of War
Call of Duty 2 and 3 (and soon 4)
Medal of Honor Airborne (first MoH I've liked since the first one)
Perfect Dark Zero (very cheap now...some places under $10...well worth it)
Orange Box (when it's released soon)
Halo 3 (of course)
Rainbow Six Vegas
GRAW 1 and 2
Time Shift (the demo is awesome, I love it)
"Meh" (I'll buy when cheap):
The Darkness
F.E.A.R.
A few not worth your time:
Hour of Victory (horrendous)
Call of Juarez (didn't like it, very plain and generic and visually last-gen)
[I know I'll get hell for this next one since this is MacRumors]
Marathon: Durandal

(I couldn't stand the demo)