I think this is another way to get us used to paying for things that used to be free. A friend of mine has XM in his car, and he is more frugal than I am. He swears it is worth the money.
By "stations" I think it is more like types of music............for instance there are "50's, 60's, 70's, 80's, 90's, metro traffic (for selected cities), country, rock, salsa, latino, classical, etc. "music stations" on XM. I think they may have different deals with different personalities for their "talk-radio stations". I was unaware of deals to cover NFL, MLB, NBA etc. I know there are "sports" stations on there, but maybe for NASCAR, NCAA etc.
I believe there is info on the web about each one. I would try home pages as a starting point. $120 per yr seems like alot to listen to the radio though. There are also some areas on out trip to work where the signal drops out too.