I personally think it’s what you’re used to, or what your gaming habits started with.
For me it’s Keyboard & Mouse for FPS games, Strategy games and some other select genres. For certain games though, a gamepad is the way to go because the game was designed for it i.e. Assassin’s Creed. Psychonauts was a whole lot easier too with my gamepad than with my mouse & keyboard.
My friend too would agree, he bought the OB for his PS3 and prefers TF2 on the PC because he found it easier to aim(not to mention hardly any FPS issues, more content and less bugs).
For racing games, I cannot stress this enough, a good FFB wheel is needed otherwise you will not get the car correcting itself which can lead to crashing/spinning out. I went from Keyboard straight to FFB Wheel and the difference is amazing, it’s so much easier to race with the wheel than the keyboard/a gamepad.
For Flight Simulators, a joystick is a must otherwise you end up… ahem… unable to fly properly.
All in all, I think it mainly comes down to if the game was designed for a gamepad/joystick/steering wheel or keyboard and mouse. Some games just play better on a gamepad than a keyboard and mouse.
I expect to be flamed now :3