A friend has it, and it sounds....ok, not all that great, IMO. The sampling rate of both XM and Sirius is too low to equal a good FM station, let alone CD. I have a fairly expensive set up in my truck, that sounds really great playing CDs and the two really decent sounding local FM stations. I would say that Sirius is slightly worse than the average stations, and XM is just slightly better sounding than Sirius (I borrowed an external XM receiver for a week, I have Sirius built in) . If you have any 128K rate MP3s on your PC, they are about equal to XM, IMO. Sirius doesn't sound quite as good as XM, IMO.
At 65+ MPH, they sound ok, in the house, ok as background music, nothing more.