I'm playing with Boxee tonight and running it through the motions and I when giving Netflix a try I was very impressed with the interface. The problem I had was that it took an extremely long time to buffer and start playing the movie. A lot longer than if I went through the browser. However once it was done buffering and started playing it seemed to do fine.
I did not however watch an entire movie at that point in time, but once it started playing I was pleased.