I just signed up for Sirius (after 3 years with XM), and know what people mean when they're talking about "artifacts" on the music channels. But, as a longtime radio dude, I'll tell you what I'm hearing seem to be AUDIO PROCESSING artifacts...such as distortion from processing too aggressively, rather than DATA COMPRESSION ARTIFACTS from the lossy compression used. Sure, I can hear those too, but as a longtime internet radio fan, they just don't bother me as much.
Other than occasional distortion, the thing that gets to me (as an audio pro...I own my own studio, and produce material for radio--commercials/liners/comedy bits/voiceovere/etc.) is the fact that the public radio channels all seem to be in mono, and WSM is in mono (I think I heard them in stereo on a boombox a few months ago when auditioning Sirius!)