Runs almost as good as on Windows, completely playable.

There may be some issues with crashing on mapchanges or joining servers, however, those issues are also present in Windows too because the game is not that well optimized (It has some memory leaks). You just need to change Texture quality to "Medium" and it will stop crashing.

Fonts on main menu can be very small or distorted on higher resolutions, but It's still legible. So far I had that problem with every source based game I ran trough Wine/Proton, so I guess it's some quirk of the engine itself.