I've owned the steam controller for roughly a year now. Through this year, I've experienced both the high points and the low points of it.
Pros:
Touchpad controls; way more accurate than a thumbstick, close to a mouse but not quite.
Rear Paddles; more controllers just need this as a default, and not as a remap button like the XBOne elite or the SCUF controllers, it allows much greater flexibility.
Customizeability; Valve themselves sell aftermarket skins, not to mention the third party vinyls and grip cases. I have a "Foamy Lizard" silicone rubber grip case on mine to counteract the grip issues, and it works wonders.
Customizeability, again; Steam has an amazing controller configurator that goes far more in depth than you would suspect. Incompatible games can be made compatible just by creating a controller profile that maps the steam controller to either M&K or other controller layouts.
Buttons; The buttons on this controller feel far more tactile than any other controller I've used, although a bit small.The click of the face buttons has a very deep tone to it that matches the feel f pressing it perfectly. Though the bumpers could have been made to sound deeper. Also, these are the first dual-stage triggers I've used since GameCube, and Honestly, Valve did DST BETTER than the GameCube; it feels more responsive, more tightly tuned compared to GameCube controllers both First and Third party.
Cons:
Compatibility; Not nearly enough games support this controller natively, and some of the ones that I've tried which do, are actually broken or easily breakable. TF2, for example, is way too easy to get the controller backwards, running the menu subprofile ingame and the ingame subprofile in the menu, with the only thing you can really do being pausing and unpausing the game. More often than not I find myself going through the configurator to make a game work, rather than just being able to launch a game and not think, given I haven't already gone into the configurator and perfectly translated the controls.
Shape/Grip; This is probably the most problematic thing about the controller for me, as I aluded to earlier. The actual grip shape of the controller is very comfortable, and I can see why it was designed this way. However, in very sweaty playsessions (literally getting hot and sweaty hands, not necessarily constantly clutching a game or anything) my controller will constantly slip down in my grip, and I have to keep pushing it back up or else I can't reach the controls with the right fingers. I had to buy a silicone rubber skin with grip points all around the palm to prevent this. With the grip case, though, I haven't had any issues.

This concludes my review for the Steam Controller. If you don't mind going into the configurator for a lot of games and finding/making profiles, I highly recommend it.