They've released a trailer and it's fairly impressive. That said, after I watched it I was a bit skeptical. Valve has pushed back it's Steam Machines till 2015 and now we hear about a Steam based handheld? Not to say I don't want one, but i'll hold off on my excitement until I hear a few more details.
http://www.steamboymachine.com/

It isn't a Valve project, so I don't factor any Steam Machines delays into this. It's a third-party who wanted to make a handheld gaming PC, basically.
And unfortunately there isn't enough data here. Does it run them natively or stream like the NVIDIA Shield? They say it can play "most" Steam games, but does that mean most current or most of the library... because a PC that's way outdated can still play most of the library, since there's such a major back-catalog.
For sure. That said, if they want to use the "Steam Machine" brand or controller they have to sign a contract with Valve which means they are on "Valve time." That's why the Alienware Alpha isn't a "Steam Machine."
That said, the idea of a Steam handheld is great might be worth waiting for. To this date I haven't picked up a handheld gaming machine because i've been waiting for the right one. That said, this may be it?
If it will let me play something like Wolfenstein: The New Order on the go (i.e. not tied to my home network) then yeah, this will be one to get, for sure!
If it's Steam proper, then they will allow in home streaming. Which is fairly similar to what the Wii U or Vita offers!
I don't know. Unless you have kids, and only one TV/monitor in the house, in-home streaming to a portable seems really excessive. I tried it on the Vita, and it was cool for all of two seconds. Then, I sold my Vita. For me to enjoy mobile games, they have to be designed to be played on mobile systems. Playing a console, or PC, title on that small of a screen is like watching the Avengers on an iPhone. It might do in a pinch, but not desirable. If Steam actually opened up to market native handheld games, that would be one thing. This... I don't know.
I agree native handheld games are the best option. But there are cases where I'd maybe want to play a PC only game on the couch in a pinch and the wife is using the TV. Realistically it's not a huge use case at all.