Yeah, I've got a Steam box but it's a bit bigger than that thing. Actually, it's a lot bigger. Got my beast of a comp housed in a nice Lian Li tower case, all black aluminium. Mmmm. Lovely!
Like alexatkin, I'm a bit sceptical about the trade off in power and performance in such a small package. It's all right for casual gamers and those who are not expecting cutting edge graphics and mind blowing physics and stuff. Not sure you'd be all that happy but as with alexatkin's links, if you really wanted a living room pc then you could research it for something a little bit bigger.
There's no requirement to have one of the Steam boxes for playing games via Steam. Actually, Valve are really branching out and as well as Mac and PC support, there will be support for Linux (which will be interesting to see how it turns out). Gabe Newell has made a lot of fuss about the Windows 8 OS and so it's quite exciting to think that may no longer actually be a requirement in the future of pc games playing. Definitely could be onto something here. There are optimised games on Steam for armchair/hand controller playing. Don't know how effective they are though. Mouse and keyboard for me!