Valve has released some interesting stats for controller use on their platform. In summary, the Xbox 360 controller is far and away the most widely used controller on Steam, with 45% of players using it over other options. Another 20% of players use a PS4 controller. Close behind that is the Xbox One controller at 19%. PS3 controller use slips in at 7%. Finally, everything else, including the Steam Controller, gets lumped together for the final 8%. In fact, there are only about 1.5 million Steam Controller users.
What does this all mean for controllers on PC? Valve seems to consider their Steam Input controller code a success. Despite the relatively low sales of the Steam Controller itself, opening up the input configurations for all controllers to user tinkering has benefitted all players. Valve remains committed to supportung as many controllers as possible.