I've had Sirius since Monday in my Mustang, and I can verify that the SQ on Sirius is terrible as well. Admittedly, I got Sirius primarily for Howard Stern, and naturally, his show sounds fine. When you throw in music, however, you can really tell it sounds terrible. It sounds like a streaming WMA file, hardly comparable to an MP3 encoded at 128kbps. Talking about the music's dynamic range is pointless; the music starts lacking in the overall reproduction of the sound in general. The highs are the most noticeably lacking among all frequencies, but everything in general sounds really bad as well.
I expected a lot better. If it weren't for Howard Stern, I'd never renew my subscription to Sirius again after this month. Sure, the selection is nice, but for a guy like me who doesn't listen to downloaded music because the sound is far from acceptable, the music on Sirius doesn't appeal to me (at all).
By the way, I'm running a Sportster 4 using a mini-RCA cable to connect the Sportster 4 into the auxillary-in on my Eclipse CD8455.