I have sirrius and love it. XM is good as well, they are basically the same.
I have the playback model, which records 45mins of what you have been listening to so you can pause the radio, or even rewind. You program in 20 of your favorite songs and it alerts you when they come on. The unit is more expensive, and larger, but can be transfered from the car to home/work easily. At first I thought I wasted my money when the basic kit is $50, but now I'm glad.
The pro's are the programming. Not only so many music stations, but the comody is great. Sirrius has "Blue Collar Radio" which is nessasary for sanity for us politically incorrect. If you like Howard Stern we have two stations of him. All are UNSENSORED.
The cons is that it takes a couple seconds when you change stations, and it takes a minute for the programming list to load when you turn it on. It's annoying when freinds who don't know the system try to find something, because they alwasys get lost. 140+ channels.