I'm skeptical and I'm going to stay that way until I actually get to try it because of the haptic feedback, but..
The face button layout also gets to me. That's going to require some serious mental gymnastics to re-wire 20+ years of face button layout muscle memory to use this in some games (edit: especially platformers).
I'm thinking Valve basically wrote off retrogaming with this device - it's after the modern genres that are completely split between the PC-style mouse-and-keyboard and the console-style thumbsticks-and-shoulders.
A retrogaming pad would be nice as an additional peripheral to sell, but realistically you can use any existing bluetooth gamepad with Steam.
Good point: Since it works w/ any gamepad, in theory, you should be able to get a nice bluetooth version of your SNES pad and have it Just Work. If you're already playing those kinds of games, you might already have one for your computer anyway.
The face button layout also gets to me. That's going to require some serious mental gymnastics to re-wire 20+ years of face button layout muscle memory to use this in some games (edit: especially platformers).