Great piece by John Comes from Uber Entertainment breaking down why a mouse and keyboard feels so much better than a console controller for playing first-person shooters:
It all comes down to how each input mechanism affects your ability to turn. On a console, the angle in which a player can turn is a function of both time and displacement of the thumb stick. No matter how far players want to turn, they have to pay a time cost. Even at the highest controller sensitivity, there is a time cost to be paid. On a PC the angle of turn is a direct mapping of how far you move the mouse. The time cost is variable and the better players get that time cost to approach zero.