I have both Sirius and XM and I have not listened to "terrestrial radio" for three years, with the exception of local Yankees broadcasts here in NY that aren't broadcast on XM (they broadcast the home team radio for MLB).
Now that SIRI and XM have merged, they have combined a lot of the music programming (for free) and other programming (for a monthly fee). At this point, they are virtually the same. The music on XM was awful (although it has gotten better post-merger), but they do carry the NHL and MLB. Sirius had better music, still has better programming, and the NFL.
Satellite radio is something that I could not imagine living without (although I obviously lived w/o it before its inception).