I got to play with an official SteamBox today for about 10 minutes; the only software installed was TeamFortress 2.
I didn't mess with the OS much - I just used it to jump into a game.
So, the OS is probably the larger part of this thread, which I can't comment on.
But I can comment on the controller - wow. I want to just come out and say "it sucks", but that's not entirely fair since it obviously isn't tuned to run TF2.
But I will say that it was very tough to come to terms with - a large part of me was screaming out "why not just use joypads" instead of the touch sensitivity bit.
It almost felt like there was a trackball under the "pads".
The physical box wasn't much larger than an XBOX One -- but it was *hefty.*
For me it's now a matter of waiting for either
a) software to catch up to the controller
or
b) an alternate controller