You'll be able to play any of the 225 supported PC games on the Shield, and even buy new ones directly from Steam and Uplay. Titles download nearly instantly, NVIDIA handles patches, and games are stored on the cloud so they don't cut into your local storage. Best of all, you'll be able to run recent titles like Witcher 3 at high frame rates and resolutions as if you were running a high-end NVIDIA GeForce card. It supports cloud saves and a single account will let you do streaming from the Shield, a Mac and a PC.
Confusingly, the new service replaces the current one offered on the Shield, NVIDIA tells Engadget. Right now, it costs $8 per month, and only works with games you already own, so you have to purchase new ones on a separate device via Steam or Uplay. By contrast, the new offering works more like GeForce Now on PC or Mac, letting you play your own titles and purchase new ones directly from the Shield.
The Shield is one of the better (albeit more expensive) 4K HDR streaming devices out there, but it was also sold with the promise of playing high-end PC games via GeForce Now. NVIDIA has now better delivered on that promise with a more complete service, and it's free, to boot. We're still waiting to find out how much it will cost when it comes out of beta, however.