I tried it out. I'm a very casual gamer. Some types of games cannot tolerate the extra latency. GRID, a racing sim, was basically unplayable. The input latency was too noticeable.
PUBG, on the other hand, was perfectly playable (although I have never played it natively so can't compare). If I were a skilled player, I might have noticed the latency, hard to say.
I'm on Linux, so I was interested to try games I can't run easily. Unfortunately, chrome on Linux doesn't have VP9, so 4k resolution was not possible. It also didn't fit my wide-screen aspect ratio, so I had black bars on the side.
You can compile it yourself or some such hacky thing, but the whole point of Stadia for me was not to futz around with that stuff.
Note that this was from Vancouver, with hard-wired ethernet on ~300Mb/s cable. If you are closer to the server (California?), your results may differ.