I liked it for Portal and that's about it. I could never get used to the steam controller despite researching peoples tips and tricks. The touchpad that is to replace the mouse, is incredibly difficult to use and I feel like the controller itself has so many customizations for tweaking it, that it becomes overwhelming. I ultimately end up dropping it and using an xbox controller that "just works".
I myself have limited time to game now days, and the last thing I want to do is tinker with a steam controller to make it bearable. But I understand some people LIKE to tinker with that sort of thing, so maybe the SC just isn't for me.
On a related tangent, I feel like the SC was made to be shipped with the steambox as their console controller. And that proton development is an effort to make the Steambox more viable and marketable. If that's the case, they need to rethink what controller they're shipping with the steambox because the vast majority of people that buy a "console" (steambox is barely a console in my mind), aren't going to want to tinker with a controller just like they don't want to tinker to get games to work on their steambox, hence the development of Proton. My 2 cents.