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It's released for general use today. I've been playing around with it all afternoon. The game runs on your Windows gaMING PC, and you can stream it over a LAN (or presumably a VPN if you wanted to be a masochist and try it over the internet) to any other device running Steam (another Windows PC, OS X, Linux, etc). Right now streaming is limited to a Windows client as the Host, but presumably any Steam client (OS X/Linux) as the client. Since the game is running on your Windows PC, you get the full Steam library (and then some, it's not limited to games purchased through Steam).
It's not limited to Steam-only games, you can get it to stream any program or desktop if you wish. That isn't really new, lots of programs do this.
In my home, over WiFi-N, I was getting 60+fps at 1080 over the LAN. That isn't too bad. Again, other programs have been known to do this, but most general purpose "remote access" programs can't do this, even over a LAN. There are still a few hiccups occasionally, I wouldn't advise anyone to play competitively, but for the most part I was extremely surprised at how fluid it was. Streaming from my Desktop to my laptop and setting them side by side, it was nearly like looking at mirrored displays - that was a very slight perceivable lag on the streamed client, but the video and input were all smooth and responsive, and it was very playable. The additional lag may be enough that if your in an extremely competitive situation it would put you at a disadvantage, but then again, if playing your in such a situation you probably aren't the core audience for this technology.
This makes SteamOS/Steam Boxes make a lot more sense, although not in the configurations I've seen being bantered around. If this is the model for SteamOS/Steam Boxes -- very inexpensive set-top boxes that can stream from a more powerful computer (similar to nVidia Shield), I can totally understand that. This is something I've been fiddling with for a very long time, and now it finally works. I spent all afternoon playing various Windows games over my WiFi LAN on my Mac Mini on my TV running OS X. It was wonderful. And if you have a gaming PC already, all you need to hook up to your TV is a old or bare bones PC, the hardware in the client is mostly irrelevant and your LAN setup makes a bigger impact.
All you really need for in home streaming is the ability to output to the native resolution of your TV. Mine ran at 60fps very well, on a Conroe Core2Duo and nVidia 320GT - which is probably slower than current Intel integrated video. Old hardware would do this just as well as anything new, really. But for those SteamBoxes with 780GTXs in them - that I don't quite understand. HTPC is nice, and I guess if you didn't already have a PC then it would make a bit more sense, but then your not really competing with a console, your competing with a PC, and your major selling point is that you don't have an OS that is required for 90% of the games....
For me, In-Home Streaming is 100x better than a console. I get to sit on my couch and be fat and lazy, it supports any controller the client OS supports (keyboard, mouse, 360/PS, Logitech, whatever), I get full access to my Steam library ~and~ my Windows gaming library, and I get to use all the expensive hardware I have in my PC to mostly it's full extent.