The OnStar and XM Digital Radio both use the same antenna located on the roof of the vehicle.
XM uses a form of compression (as mentioned previously) that leaves a bit to be desired when it comes to music sound quality, for talk radio it is not a problem, but it should still sound fairly decent.

The XM digital radio receiver (DDR) is a separate unit located under the dash, if you are experiencing lots of drop-outs or very poor sound quality that cannot be traced back the antenna or cable’s then you should have the dealer look at the DDR unit.
Also make sure that your stereo has the latest software calibration, there was an update back in June for the Black-tie (non-nav) units and the current Nav radio calibration is 15260985.

 

I quit XM. Sound quality was very poor due to too much compression. Normal FM and CDs are far superior. Far superior in sound, that is. I have the 2007 LTZ with Premium Bose.
Secondly, I didn't care for all the commercials (every 10 minutes) on the non-music channels - and some music channels. Thirdly, the programming on XM is very inferior to Sirius. No NFL, for example.

Bottom line: I think GMC made a huge mistake with the entire Navigation package. They should have gone with Sirius and decent mapping software - such as used by the Garmin Street Pilot. Although I have version 2.0 software, my unit still tells me to go where no roads exist! Yikes. (I live in Arizona) In addition, the software is very clumsy and non-intuitive.

I think they tried to save a buck and got burned. Some VP should be fired. I sure hope somebody it taking notes.

 

I tend to disagree, I'm quite happy with the way my xm sounds, I have the nav radio and that may have something to do with it, but I feel I'm a fairly discerning listener and so far I've been happy with the sound quality of my xm.

 

I just recently changed out my Stock radio for a Nav/DVD GM model. I was very upset about the quality of both xm and the bose all around. After I put this new head unit in, it all changed. The XM now is very close to cd quality, or at least 75% better than previous, and it does sound as if this unit "releases" the subwoofer as now it really is prominant and tight, not muddy as before.

However, my rear seat entertainment is dead in the water as a result of the swap, I'm praying it just needs to be enabled inside the radio by Tech II, waiting for verificationb however... Anyway, sound quality improves substantially with the -66 head unit.

Happy Hoe-ing

 

Having current personal knowledge from use of the nav systems in my Lexus and BMW, I feel the GM Nav system in my 07 LTZ is quite comparable. I would rate the Lexus slightly more user friendly than my GM Nav, but it beats my BMW. Just my humble opinion.