I just installed sirius around 2 weeks ago, and I am falling in love. The sound quality, while not exactly CD quality, blows FM off the map. However I did connect it directly to the head unit, I'm not using that fm transmitter crap. I lose the signal for a second when I go under an overpass, or in a gas station under that metal roof, other than that, it has been fantastic.
I have also found alot of stations that play stuff I like. I'm more into the classic rock, hard rock stuff. I think the selection is pretty good. Commercial free is a nice bonus. There is some self promotion stuff, shows on other stations, ticket giveaways, stuff like that, but no 10 minute blocks of advertising. All in all I think it's worth the money, 20 years ago no one thought people would pay for TV either.
P.S. they both have free online samples that you can listen to for a couple of days and see which one has a selection more suited to your tastes. Good luck.