The most interesting tidbit (for me at least) that Alex didn't mention is that this will work with every steam game, even those that don't have controller support, since the steam controller is recognized as a keyboard/mouse, not a game pad.
The Steam Controller is designed to work with all the games on Steam: past, present, and future. Even the older titles in the catalog and the ones which were not built with controller support. (We’ve fooled those older games into thinking they’re being played with a keyboard and mouse.)
I think this is pretty awesome, really want to know how well it works.