Well this thread is a fucking cluster fuck now isn't it.
Tho it looks like you have mostly made your decision by now I might as well clarify some things.
So SteamMachines is an initiative from Valve to give Steam users PCs ready for the living room. Which may sound like what you want in theory, but it comes with a lot of caveats. As others have stated as they by default will run Valve's SteamOS which is a modified version of Linux. And the trouble with that is right now there is very little support for Linux in terms of games right now. There are however some signs that is changing a bit. Valve are putting quite a lot of resources into getting developers to support the platform, including providing tools for porting etc. Many of the major 3rd party game engines are adding support for Linux including Cryengine and Unreal Engine 4 as well as Valve's own Source Engine and the ubiquitous in indie circles Unity.
Now this doesn't mean that games will automatically come over but it will make it easier for especially smaller developers to bring there games over. As for the larger publishers they have mostly been cautious so far, and in terms of AAA games there haven't been any major announcements so far. However there have been some smaller announcements from major publishers for example Warner Interactive announced their new Gauntlet game will support SteamOS, Deep Silver put out Metro: Last Light on Linux. These are baby steps tho and it remains to be seen how much steam (pun intended) it picks up in the big publisher space.
SteamMachines and SteamOS are both long term bets for Valve and they will be evolving their strategy over time. Valve don't operate like the big three, they pride themselves on being adaptive and forward thinking. It remains to be seen how it pans out but it will be more like Steam's launch and gradual evolution than a traditional platform launch. And honestly I can't see it being a good consumer friendly major platform in the next couple years. And it's worth keeping in mind that SteamMachines are still PCs, you will likely have to do some tweaking to get it working how you want anyway. And you will still be running PC games, some of which may require a little tweaking to get running just right.
Now the option of just getting a suitable PC with a living room friendly case is one worth considering IMO. I know you are pretty set against it as it will require a little bit of effort (it's not that hard, but does require a bit of time) to get it set up. But if you were willing to put in the effort you would most likely be able to get it to work how you want it too. And if you are running on Windows you will have access to the full library of games. Setting it up to boot straight into Steam big picture (Which is the same gamepad compatible interface as SteamOS it's just part of the client which can run on Windows or Mac) is perfectly possible. This article explains how to set that up http://lifehacker.com/how-to-roll-your-own-steam-machine-with-windows-and-big-1510663145 as you can see it's not terribly complicated.
There are some drawbacks, mostly in terms of cost if you want a decent system in a good compact case prebuilt, tho the SteamMachine looks like it might actually help in that regard as it is pushing vendors to develop good compact systems and I am sure they will offer the option of just getting it with Windows which you can then configure to run Steam Big Picture natively. And of course you will probably have to do a little tweaking of settings on some games to get them to run optimally but you would have to do that on SteamOS as well, again it's not that hard especialy these days most games will work fine out of the box or with just a couple minutes or less.
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