I just bought the steam controller and steam link yesterday since gamestop had them for $35 each. I only spent about an hour or so with it, but I ended up liking both the controller and steam link more than I thought I would.
I was playing Portal 2, which I thought would be a good feel for playing an FPS with the controller without being a fast reaction based shooter. The touchpad felt very odd at first, but even after just an hour I felt I was already getting used to it and liked it better than another analog stick. This might also be do to the motion controls that are built into the controller that I turned on. That let me do slower, more precise aiming by tilting the controller slightly and only making the big moves with the touchpad. Not a bad way to go IMO.
It's not the most ergonomic controller by any means. I was trying to find a way to comfortably hold it and I was still fighting with it. If I hold it like I normally would hold a controller, I couldn't really make use of the extra buttons on the bottom of the controller where your ring & pinky fingers rest. If I adjusted to make sure I could use those comfortably, then the face buttons became more of a reach. I feel like it was designed for larger hands than I have.
There were also some software problems with switching control profiles in game. I'm not sure if it was just related to this game, or a larger issue, but every time I tried to change the controller profile it's like the game would no longer recognize the controller. I'd have to shut the game down via the steam menu, then restart the game for the game to work properly. Quite annoying
Still, it's very limited experience. If it wasn't for the controller profile issues I'd be pretty happy, and that's a software issue. Hardware wise I'm more impressed with the touchpad than I thought I would be. The haptic feedback is nice and the pad is large enough to use properly.