I have XM, one in each vehicle, Xterra and BMW325i, and one portable that I take on trips. It even works in flight if you have a window seat on the south facing side of the aircraft.
I do not agree with Eclypse20 about who chooses what. I am 42, listen to XM for variety, mainly the techno channels. I hate talk radio. XM DJ's are allowed to play whatever they want whereas Sirius DJ's are monitored by a programming director according to a news magazine article I read. Sirius plays more mainstream you hear on FM, XM plays more variety. Sirius seems to sign with deals that don't make sense, (i.e.Cosmo radio, Martha Stewart radio)XM signs with Baseball, MTV, NHL,and will soon be available in Canada.