I got a free Stadia controller when Google thought Stadia would be a thing, which went unused for Stadia's entire existence. I used Google's Bluetooth conversion site once it became available, and it works great. This isn't a review, but it's actually a damn fine controller.

I read some docs and there's something called "Tandem Mode", which to my reading means you can plug any arbitrary controller into the Stadia controller, and it'll act as a second controller. I haven't used it yet, but that's pretty damn cool.