One controller for one game which I can't play that much.
Games Chrono says a controller is good for:
Beat Hazard
Sonic games
Dynasty Warriors games
Specifically legal emulation or other console ports
Some people have enjoyed with a controller:
Awesomenauts
Audiosurf
Spiral Knights
Secrets of Grindea
And hey, some other games offer native controller support. And if you have a PS3 controller and a USB charger cable for it, then you're already set, you just gotta download Motioninjoy.