I drop all my premium channels(HBO, showtime...) when I subscribed to Netfilx, that was almost 2 years ago. The main reason was that the sound quality on DVDs is much better. I used to set HBO in DD5.1 when available, but I notice that the sound was too compressed.
I was wondering if now with HD programming, DD5.1 sound quality has improved as well.

Depends on the show. HDNet has very good DD programming. Some HBO programs are good. ABC and CBS are also good.