I have my code, but not my founder's edition, so I've been playing through a Chrome browser.
I've only played Destiny 2. So far;
Cons: The graphics are not as good as playing locally. It's like the settings are turned down.
Pros: Otherwise everything has been great. It is really fast to get into the game, load times are better than they are for me playing locally (though I don't have a SSD on my PC), lag is (to me) imperceptible, frame rate is solid. One of the things I wasn't expecting to like so much is how QUIET my PC is while running a Stadia game. For local games, my cooling fans kick on and of course that doesn't happen on Stadia. My biggest problem now is it is so tempting to just fire up Stadia for a quick Destiny 2 break. I don't have to shut down other programs I'm running to devote CPU to the game. I just open a browser tab and start playing.
For reference I'm in NC, USA, on Spectrum internet, 200/20 mbps and I've been playing with a wired Ethernet connection.
Also for reference, I'm not an eSports Professional. I play games to have fun so I'm maybe not as demanding as some folks who take games a lot more seriously than I do.
I'm used to be playing CSGO kind of competetive with 144hz monitor and graphics cranked down for minimum latency and i do notice the delay in Stadia. I wouldn't suggest Stadia for someone who is looking at Stadia to fully replace their computer if they're playing games competetively like CSGO. However, for the majority of people who are just casual gamers Stadia is seriously the next big thing in my opinion. The mouse movement in exodus & destiny are abit laggy for me because I'm used to 144hz with 1ms delay to my monitor. But when im sitting in the couch with a controller? I'm laughing because how good it is lol.