I'm an XM sub, and can honestly say that if sound quality is a top priority for you, don't get it, nor Sirius for that matter. The SQ is absolutely irritating to me when I try and get into any song which I am familiar with. The highs are absolutely ssssssssibilant, drum snares, crashes and such are just a disaster, the mids are atrociously absent, and the bass is muddier than a pig in a sty after dinner.
I mainly have my Myfi unit just to listen to a few talk shows (202 OnA RnF), but am "considering ditching it to save the cash, since I can actually download those shows if I choose to via Bit Torrent. RARELY will I turn to any of the music stations, since I tend to get annoyed after a while.
I will say one thing which is a redeeming quality for XM though: I've been able to record a slew of artists which I was not familiar with, and have been turned on to. It's a great medium for discovering new artists, though some may not be into that kind of "deep" playlist. The only channels which I CAN stand, are the classical channels. Strings and horns come across decently enough, as to not make me turn it off.
Also, something kind of cool: If you tune any radio in your house to 88.3 Mhz, it will pick up your XM signal, and broadcast it. So when you're say, in the bathroom and you have that lil' radio on, you can actually get your XM to play through it.
On the whole though, I'm thoroughly unimpressed with what they're doing with the technology. What the hell is the sense of having 150 channels to compete with another company, if you're going to deplete all of your bandwidth, and make the broadcast sound worse than FM radio ?
The only hopes of either company improving this problem, is if Sirius get what they are currently begging for, which is a merger w/XM. I hope it happens actually, because the way both companies are going, their profiles are so low, and below the consumer radar, Sat radio will be a ghost in a matter of a decade IMO.
P.S. for more XM info and a good forum (if you steer clear of the idiots whom visit from the Sirius forums to troll) try xm411.com