SteamOS has been available for a while in a less-than-complete state, for those who want to try it out and provide feedback to the developers. But just recently, Valve has unlocked the two most in-demand features: dual-booting, and streaming. The latter can be used as your own in-house cloud gaming setup.
While many were waiting for the dual-boot functionality to arrive to they could try the OS out without dedicating an entire system to it, I've personally been on the edge of my seat in anticipation of making my own cloud gaming setup. Of course, it's not really the "cloud", but SteamOS allows you to stream gaming from your main PC to another system in the house. If you've got a TV in a separate room and a Steam account full of title that work well with controllers, like me, then this can be a very good thing.
Considering people are currently wondering whether to spend money on a Xbox One or PS4, a cheap, self-made Steam box could be a much better option. Provided you're already a member of the glorious PC gaming master race, one could build a Steam machine quite cheaply, and just stream games to the small box connected to the TV in the living room. Not only would the hardware be cheaper, but games on Steam cost significantly less.
The downsides? Gaming on the actual SteamOS requires games that work on linux, though that library is ever-increasing. And while you can play any Steam game from your PC over stream, due to the extra links in the chain, there will be a small amount of latency to deal with. There's a hands-on of the streaming over at PC World. I wouldn't play twitch FPS on it, but then again, that's something best left for the PC anyway. Whether you try fast-paced action titles with 3rd-person melee combat is up to you, as I imagine that's right on the border between feasible and frustrating. But turn-based strategy? Casual indie? Everything else? Absolutely.
Due to the ARM nature of the Raspberry Pi, I don't believe it would work well with SteamOS, but I'm happy to be proven wrong about that. What I am certain of is that one could make a machine for considerably less than the cost of a new gaming console, let alone one of the official Steam machines from 3rd party vendors. I'm excited to see what people come up with.
You can download SteamOS with its new functionality here.