Mine came in today, and while I haven't had that much time with it yet, I've been trying a few things with it to see what I can make it do.
First off, aside from development, I wanted it for one thing. For years, I've been trying to find a way to play Fallout 3 in the bedroom.
I tried tablets with remote-software, a gaming laptop, the Surface pro with an XBOX controller, and even used an XBOX360 with a small TV, but it was always a mess, either with cables or just fiddling with stuff in order to get it to work.
With the Shield, all that is in the past. It works!
1. Stream unsupported games:
Fallout 3 isn't on the "supported" list, and isn't recognized by Geforce Experience.
To play/stream unsupported games on the nVidia Shield, just select the plain Steam entry in the Shield menu, and you can select your game from the Library there.
Things to keep in mind:
Geforce Experience applies different settings to games when you stream them. With games that are not supported, you will have to use settings that work with the Shield.
The screen resolution has to be 1280x720 and the application has to be set to full-screen. My desktop runs 1920x1080, so my first try of Fallout 3 resulted in a crash. Changing the resolution for the game in its settings fixed the issue.
2. Stream non-steam games AND Windows Applications:
You can add games you purchased through different means to your Steam Library by selecting Games > add non-steam game to your Steam Library, and they will be accessible on your Shield.
But, it doesn't have to be a game!
You can add applications the exact same way. I tried Windows Media Player, Photoshop CS6 and even DVD Catalyst, and all 3 were nicely streamed to the Shield.
Unfortunately, if the app doesn't have controller support, you can only control it using the touch-screen, and for some reason, on mine, the mouse arrow is a bit off from where you touch the screen.