I've just seen Paul's Hardware's video on the Steam Controller and needless to say I am not taken away by it.
I'm interested in what it is like for racing games as that's my primary use, so please keep me updated TwsT.
I think it is something that you would have to use for a month to make a proper decision.
For racing games you are going to use the stick or motion control. I do not think it will be much different than what is currently out there.
It is basically a compromise so you can play certain games on your couch and have a decent level of control.