Hey,
A trucker friend of mine has XM in his big rig, he SWEARS by it. The biggest benefit is that you can get reception no matter where you are in the states, as well as having the specialty programming, like comedy, international news, talk, and genre-specific channels...and the sound quality is near to that of cd. It really does sound great when you a/b it with a strong FM signal.
It costs something like $100 to get a receiver, and the subscription is about $10 a month, which is well worth it if you're into that kind of stuff.
What I would like to see is the ability to have local channels have access to the technology, so that for example I could listen to my favorite talk channel in Phoenix no matter where I drove in the country. I wouldn't even mind if there were commercials, and as it's only talk the sound quality wouldn't matter so much.
Forget about having something convert it to mp3, it would be cool if you could turn the digital stream into a wav file, then edit the stuff you don't want out before burning it to cd (or compress it to mp3 if you desire). You may be able to record the audio directly into a sound program like CEP or SoundForge if your card is set up correctly.
Take care,
Nick