I've had XM for over a year now.  I love it.  I can't listen to normal radio now.  I've just had a dash top unit.  Now I just recently got a new car that has Sirius in it.  I've only heard Sirius a few times before in a friends car.  I thought I liked it better than XM, but now I don't.  XM is great if you like sports.  I mainly listened to the electronic station BPM and the alternative station Ethel.  I really liked Ethel.  I heard a lot of new songs on there.  There doesn't seem to be a Sirius equivalent in one station.  Now Sirius is great for it's electronic stations and it's heavy and classic rock.  I miss Ethel though.  I think I need to just listen more.  Both are way better than standard radio.  But right now I think I like XM better.