After reports that Valve had received shipments of the accessory, a Steam Controller review has appeared. Deleted ahead of a likely release date update, a YouTuber praised its responsive dual trackpads. Even so, the price may make it more difficult to discount the Steam Machine.
Linux hardware support is excellent given that 8bitdo is the usual recommended brand. The software part is the one lacking, but you can easily configure it via steam. I have one on Linux, I game without issues.