All the main functionality works. There's an error dialog regarding the missing C:\ProgramData folder that doesn't appear to affect any other functionality (though it does pop up there error on every launch). The dialog boxes sometimes truncate prompts. The built-in screenshot function doesn't work. The manual menu entry doesn't work, but you can access it via the steam page link or their home page.

If you create a ProgramData folder manually in steamapps/compatdata/760920/pfx/drive_c/ you won't get the error anymore and Proton will stop trying to reinstall the VC runtime.