I have XM in my car and at home. I listen to talk radio, news and comedy more than anything else, and as such, can't really speak to the repetitiveness complaint that Jimmy mentioned. I've always heard people that have had both XM and Sirius say that Sirius is far more repetitive, though.
The commercial-free nature of many of XM's channels is one of the best things about XM. The best thing, however, is the ability to drive all over the US (and Canada too now, I believe) and still have the same radio stations and content available to you. Even before there was supposed to be XM in Canada, it took me 2-3 hours of driving north into Manitoba before I would lose a signal. Awesome.
Sound quality is my biggest complaint about XM. I first got XM in 2003 or so, and the sound quality was much better back then. The more channels XM adds, the worse the sound quality is. It's better on jazz and classical stations than on many other channels, especially the metal and hard rock stations like XMLM and Squizz that I sometimes listen to, but it's still not as good as it could be.
Clint, for what you want to do with XM, I recommend checking out the stereos available here:
http://xmradio.com/shop/plugandplay.xmc
Any of the portable options would also work for your purposes.