It really depends on the song, channel, and the equipment. XM sounds so compressed through my Ue-10 PRO because they reveal all, but it sounds very warm and rich through the small mono speaker of my Tivoli Model One. So, it depends.
I'll say it again that the programming content is simply unmatched. No AM/FM radio station or Internet web radio can match XM. Sirius can barely compete with XM on the same platform. Yeah, take that you Sirius subscribers! Nah. Just kidding. Sirius has its' own strengths (like talk radio, NPR, I guess that's one...ooops, no political rants allowed) too. I still listen to Sirius and the programming is very nice but it doesn't really have the XM depth of programming or breadth of coverage in each channel.
The classical channels on XM sound so good though. Barely noticeable from bargain classical CDs from DG, EMI, etc. Very nice sound quality and excellent programming especially on XM Classical Pops.
Man, I think XM is worth my money. Every time I listen to XM, it's usually for hours on end with nothing else occupying my attention. That's worth something nowadays.