I spent roughly 10 minutes with the final Steam Controller at GDC 2015, playing snippets of Middle-earth: Shadow of Mordor, The Talos Principle and Unreal Tournament on various Steam Machines. The body of the controller is wonderful to hold. Two long, clickable pads running along the backside of the handles, right where a player's middle and ring fingers lie, would be a welcome addition to any existing gamepad. Plus, the final controller adds a single analog stick on the left side. This makes the design more familiar overall, but with a trackpad replacing a second analog stick, the final Steam Controller remains what it always has been: awkward.
They say that the pads at the back of the handles is a wonderful thing to have on the controller. In general they really liked it, except for the fact that the button placement on the front are usually where a right analog stick would be. To those used to using typically controllers from the past however many years now, it may be a bit awkward to use for a while as you adjust.
A few of their images can be seen below, but the full gallery and longer impressions are at Engadget.
This controller will probably be good for something.