Keyboard + Mouse + Gamepad = Steam Controller
Valve was kind enough to show how the Steam Controller would work in Portal 2. Basically, it comes with dual trackpads, a touchscreen, haptics, a few buttons and shared configurations. Each and every one of these is employed when playing a game. Of course, at first the Steam Controller might seem a bit too sensitive, but in a few minutes, it should become natural. One of the big questions is how will it work with PC RTS games that make use of the keyboard and mouse combo extensively. In first person shooters and arcade game there won't be any problems, most probably.