Xbox One Game Streaming has been switched on for all Windows 10 users and I gave it a quick try.
You need the latest Xbox app of course, as well as a wired Xbox controller (I used a 360 controller). Turn your Xbox on, run the app, make sure you are signed in with a Microsoft account that also has a profile on the console, and hit the Connect button near bottom left.
Once connected, click Test streaming to verify that you have enough bandwidth, or Stream to start streaming.
I tried Dark Souls 2. It worked pretty well, except that the screen (which is dark anyway for this particular game) was even darker over the streamed connection. With the in-game option set to maximum brightness it was playable. I did not notice any lag though there was an occasional stutter; this is over wireless though and a wired connection would be better.
While you are streaming, all the action displays on the TV connected to the console itself as well as on your PC.
Streaming only works full screen. If you are streaming and press the Windows key you get an odd effect where streaming continues in the background but the Start menu displays.
If you then run another application, the streaming pauses, but you can resume later. Or sometimes the Xbox app seems to quit. It seems best not to switch apps while streaming.
I noticed that trying to start streaming when the console is on standby does not work; you cannot wake it up. That is a shame.
Otherwise, it all works as you would expect. Although it is not as good as gaming in say the living room with what is probably your biggest screen, this is a nice feature for when that room is in use and you want to sneak off and play a game.