Oh yes, playing an FPS is far much easier on the PC than on consoles, but I was just talking about strictly consoles.
You say that but it's somewhat subjective with mixed results, I've rocked socks with a pad on Counter Strike and TF2 and yes on a PC, I played Halo PC for quite a while with a modded Xbox Duke pad(the old HEUG LEIK XBAWKS ones) and did quite well, Lost Planet Colonies edition has cross platform PC vs 360 play, most 360 players were kicking ass and taking names on that one, Shadowrun was also dominated by 360 players but only because PC users were nerfed a bit with mouse acceleration/smoothing or something.
I usually plug in my 360 pad to play L4D2 though, and I'm playing FEAR 3 with it as well.
Fighters like Street Fighter or Guilty Gear almost always need a pad or stick to play on PC, although I do know a guy that plays PC SF4 with his keyboard as Gen and rarely looses.