As someone who spent 15 months in high school (pathetic, I know) playing FPS PC games more than 8 hours a day, I am really impressed in how the 360 multiplayer and FPS gameplay is.
A controller will never be able to compete with mouse + keyboard, but while playing COD3 in my friend's dorm I really didn't find myself comparing.
It's difficult to explain, but it seemed to simply be a different experience.