Valve has unveiled the final make of the Steam controller at GDC, along with the launch lineup of Steam Machines which will be available Fall 2015. There's a Steam Store page where you can check out the boxes that will be available.
It's very similar to the lineup we saw at the initial unveiling, in that the manufacturers are still delivering incredibly overpriced pre-built machines. The low point to me is the $600 Gigabyte device with an integrated Intel graphics chip - in benchmarks, it runs games at about half the resolution as the current gen consoles. There's also Falcon, who's machines go up to 16GB DDR3 and a 4790K but at a starting price of $2,000, it better.
That said, some of these aren't too bad. Alienware, iBuyPower, and Syber each deliver boxes that approach console pricing, with GPUs that mostly keep pace with console benchmarks, although some might fall a bit short. My favorite is the ASUS GR8S - $700 for a GTX900 series card and a killer form factor, although the memory capacity might be a concern.
If you're someone who games on a laptop and curious to see what a more dedicated PC experience is like but are too intimidated to build your own, you could do a lot worse than some of these. For everyone else, you really are better off buying a console or building your own. Also keep in mind these boxes run Steam OS, so a game will have to be available on Linux to run natively. There's a Steam Sale right now showcasing what that library looks like, it's actually already very substantial.