I played for several hours on a Mac client yesterday. It seemed very stable as long as you only ran it on the crappiest graphics setting. I have a top of the line Mac with super graphics cards, but when I ran it in better graphics modes, it crashed trying to load all the textures and meshes. I gave Richter my crash logs and he now knows what is causing the crashes. No ETA on when he will get them fixed, but rest assured that I will be a Mac testing machine helping him get the client into tip top shape.
So we have a working Mac client in-house, but only in the worst graphics mode. Not quite ready for prime time, but close.
-Lisa