XM is a great service with some major issues-- I've been a subscriber for about 3 years now and the sound quality has gotten pretty crap. It actually wasn't tooo bad (you wouldn't mistake it for a CD but it was perfectly acceptable) until the beginning of the year, when they started adding channels like crazy... most of the XM music stations now broadcast in the 32kbps-48kbps range, which sounds pretty bad. If you hear a song you love that you have on CD, it may be virtually unlistenable, but it does depend on the station. (Rock/pop tends to get lower bitrate than jazz/classical, for example)
I've become pretty disappointed with XM lately due to this. If you have DirecTV, the XM feeds through that sound infinitely better than what you'll get with a radio. I cancelled my Roady subscription when it came time to renew it cause it was unlistenable in my car. My home setup is a bit better but it's nowhere near as good as it used to be.
Basically,depending on the station, XM sounds somewhere between AM and FM right now. It rarely exceeds FM quality (and yes, I even mean plugged into a home stereo with direct wires, I don't mean the FM modulator). I have kept the 1 receiver active as I do like it, but when XM Online sounds significantly better using 64kbps WMA than the radio I paid for, there's a problem.
Granted, $40 isn't bad, and for some types of people it's a GREAT deal.. but don't go into it thinking you're getting CD (or even 128kbps MP3)-quality sound. It'll sound like dial-up internet radio. 
I've noticed my listening patterns have gone from all the time at home and in my car, to now mostly only at work, listening to XM Online-- I can't handle the quality drop of the radios anymore. :/ But if you're in your car a LOT, if you want a lot of sports channels, if you're not picky about sound quality (Think of it as quantity over quality), it's a great service. I'm holding on with my one subscription hoping it gets better again..
(and apparently Sirius has its own quality issues, so it's not like satellite radio is where you should go if you're an audiophile)
