I've spent four hours with the Steam controller so far, which simply isn't enough time to master this new device. Your experience with mice, touchpads, gamepads, keyboards or any other input device simply will not carry over to your proficiency with this gamepad.
It's not going to replace the mouse and keyboard for competitive first person shooters (Quake / CS:GO) and I probably wouldn't try to use it for competing in RTSs either, but for games where an XBox controller would be used it's flatly better than other gamepads.
I've been going through Doom 3 with it and I'm getting the hang of it the more I go along. There's something strangely fun about playing games with this controller. It's built well and the haptic feedback is excellent. I played a round of Vermintide with it and was able to hold my own.
I dismissed the motion controls out of hand, but after watching a few videos on Youtube, I've found that it's a great addition for fine aiming (e.g. trying to keep Q3A's lightning gun on target).
The bottom line is this: if you're going to get a gamepad for PC gaming this is the one to get hands down. My Xbox controllers will now be collecting dust. If you want to use it in place of a mouse / keyboard there will be a considerable learning curve, but overall the controller is more fun than you would expect to use.
I give it two thumbs up.
- Z