Works out of the box without problems. Playing with friend (he is on Windows) in coop, no major problems. Some notes:

• We experience minor freezes when traveling from one location to another, but I think it isn't Linux-related problem;
• We use only in-game voice chat, it works perfectly;
• Sometimes the game utilizes my GPU too much. For example, in some portal worlds GPU utilization could reach 100%. It doesn't affect performance much (FPS are still about 60 with VSync and temperature is ok), but nevertheless. Graphic settings are low-middle