Fenyx Rising demo is up. The demo is Stadia exclusive, but free for anyone to play. Going to give it a go this evening. Will be an interesting test of both the game and the service.
It turned out to be very interesting.
GOOD :
The quick set up of a new account
The instant start of the game without any downloads/patches/configuration
The game itself was fantastic - really looking out for this now
Amazing when you think it's running in a browser
BAD :
Performance - despite having excellent connection status in the overlay, the game was a stuttering mess. Whether this is due to Fenyx only being a demo or me using Microsoft Edge as the browser instead of Chrome, I don't know. But as that was my first experience of Stadia, it's not very good. I'm going to have a look and see if there is anything free I can test further with.