I think they started using their proton layer for dota2. I am lately experiencing lags during their heavy animations when presenting Arcana and special game modes, other than that it works fine. I have a lot of games since I buy 3 year old games for 5 dollars and most of them work fine. The newest games with special anti cheat systems probably do not work.
But this is a huge step. They are ahead of the competition. If the competition wants to offer something better than Steam they now need to invest heavily in linux support which increases the costs of development significantly.