I have had XM for a year and a half now and couldn't imagine being stuck in the car without it. It is amazing compared to the local crap, with no DJ's on any of the channels I listen to. The only thing that I don't like about it is the fact my 8yr old takes over when she is in the vehicle and it is Radio Disney only!
I can also get access to alot of different stations that aren't available here in Canada, from sports to news/talk. Since I had it installed I have had many friends/family get it. I also got the home kit and use it indoors frequently.