It’s not a unplayable, but there’s definitely noticeable latency / lag compared to playing something locally. The resolution is also lowered compared to a decent midrange PC. Obviously, this depends on your internet connection and what you’re streaming to
The resolution is also dependent on your subscription. Without Stadia Pro, it's 1080p max.
In my experience some games are just very laggy, where others appear "optimized" for Stadia network. I can't put my finger on it, but my comparisons come from playing the same title on PC/Steam. Cyberpunk seems to have none to very little lag. And that's playing with the chrome cast device on my wireless, Chromium on Linux via ethernet and Chrome on macOS. All platforms work great with Cyberpunk, but Destiny 2 is another story.
None at all. I've had a few action sequences in the story now and they all went smooth. Sometime quality notably drops, but not pace/framerate. The one time I had an issue with frames dropping was because a local computer downloading files and causing bandwith spikes. But I think with some Qos on the router that should be fixed. Average bandwith usage of the game is between 10-20Mbit.
So I just played Celeste, a platforming game which I have both native and on Stadia (it's free with the Pro subscription) and I don't notice any difference in playing either version in terms of input lag.