I gave it a shot with a PS3 controller, and it really is quite nice. Unfortunately, it still has a few quirks (updating killing floor popped up a window which could only be closed with the mouse), and a lot of games just don't support controllers period. The killing floor server list, for one, cannot be navigated with a controller at all.
It would be nice if they had a list of your games that were "100%" controller compatible, before you launch it on accident and have to get up to close it with your keyboard.