I linked my account that I also use on Steam, that worked well. I can now login from my PC(Steam), Phone, or the Chromecast. I did have to purchase the base game again, it is $19.99 on Stadia but I got a discount to $9.99 so whatever. You DO NOT have to re-purchase any DLC or expansions you already have if you are linking an existing account to Stadia, don't do it!!
One thing I like to point out is the control settings, if you have custom keybindings on PC they do carry over to Stadia, even though there doesn't seem to be a way to modify those keybindings on Stadia except the default template layouts. I need to change a few because I have no way to open my map as its bound to the keyboard and Stadia doesn't recognize mouse + keyboard. I have to figure what I don't need, probably get rid of sheathe/unsheathe toggle for maps.
Add-ons... obviously don't work, since the game data isn't saved locally anywhere. The default ui is dreadful, I miss Azurah and Srendarr customization less than 5 minutes in, the default displays just can't cut it. Graphical settings seem to be based on whatever Stadia wants, so I lose out on my custom 165 FPS down to their 60 FPS. Shadows, I hate them, and they're turned on higher than what I'm used to.
My plan? Sometimes in the middle of the night or when I'm bored and don't want to turn on the PC, I can turn on my phone and start playing immediately to do a little farming, some questing, or goof around. It works... kind of, 2.4ghz connection doesn't seem to work, no matter what, even sitting next to the router even though everything else running works fine. There's still the signature Stadia random connection drop mid game where you spin uncontrollably or button lag happens. I would NOT PvP with this or do group stuff like dungeons, its unreliable. Overland its okay, just prepare to die often when the controller stops responding even on 1000 Mbps up/down wifi.