I bought, but later returned, the pre-release version of one of these, the Alienware Alpha. It runs on Windows 8 with an Alpha GUI wrapper around it that let's you access settings and modes via a controller. From there, you launch into Steam Big Picture mode to access your steam library. It comes with an XBOX 360 controller.
It was sort of an impulse buy, got over $200 off on it. Once I realized there were not many PC exclusive games I wanted to play just yet, I decided to return. The one title I want to play someday, Pillars of Eternity, I can play on my Mac and requires KB/Mouse. Everything else I can play on my console.
The hardware itself was really nice. Smaller in size than xb1/ps4. Decent parts, but graphics card is not up-gradable so you'd be stuck playing medium to high settings (no ultra), and might eventually not be able to play more graphics intensive games that come out in a couple of years. The software GUI wrapper felt useless, as you really just need steam big picture mode. Install was not straightforward and I needed a KB/mouse to get working right, had to download drivers, and some games did not install or work right. Yep, pretty much what you expect from a PC...
Eventually, I might see myself getting one again, only to take advantage of the much lower prices on PC games. I'm so envious of those Steam Sales! But I think if I go gaming PC, I want to go whole hog and build my own with a high end user replaceable graphics card.
As Foobar stated, I don't see the steam OS taking off, as not enough games are linux compatible. If you get one, I guarantee you'll end up putting windows 10 on it. The jury is still out on the Steam controller too, no telling if it's any good or not until these things release in the fall.
