I switch pretty easily. First I played Rayman games with keyboard + mouse, then a joystick (...yeah), then after 3 months of no play I tried again, too hard, then keyboard + mouse, then gamepad, then keyboard + mouse, and finished with gamepad.
Same goes for Jade Empire. I think I wouldn't want to play it again with keyboard + mouse. Gamepad is good for that. Or Devil May Cry. Assassin's Creed, however, is much better on a 2000 dpi mouse.
Oh, and it took me merely 30 minutes to switch from Mouse aim to a Xbox 360 gamepad aiming in Army of Two. Which is strange, because I couldn't do the same in Gears of War.