It's not magic, it's an Nvidia server you're paying for on a time share. And it's decent, but frankly, as the kind of person that can tell when my 120Hz VRR display is hitting a flat frametime by eye it's nowhere near comparable to local play, even in optimal circumstances.
Streaming is a nice option when you need a hardware-independent, location-independent way to run a heavy game, or as a stopgap when your client hardware can't cut it with a modern release the cloud service covers, but it's not an optimal experience and it's problematic if it becomes a primary way to run games for a host of other reasons. I actually find GFN to be a solid idea, in terms of tapping into libraries you already own, but it's absolutely a secondary, value-added solution to either running games on client or even pushing your own stream from a server you own.