Okay, everyone talking about the people with $200 mice and other expensive equipment... give me a break. I know people that can dominate people with top-of-the-line equipment (this goes for PC and input-equipment alike) but run lagboxes with ball mice and old, crusty keyboards. There is a difference using a better mouse, don't get me wrong, but it doesn't magically make you a better player by any means.
IMO, I can't stand controllers for FPSs. I can't stand Halo at all either. It's not something I ever got into in the first place, but it isn't too appealing to me over other FPSs. Shooters belong on the PC, IMO. CoD2 is a great 360 game though. I'd probably think it's better on PC if I had ever played the PC version. Other shooters, though, are console games by far. Metal Gear Solid series, Splinter Cell, those are all console games to me. There's no comfortable control scheme on PC for those games IMO.
In addition, find me a PC FPS that has a subscription service