I had XM at home for about 2 years. I loved the music and hated the DJs, so when Napster came out with the $9.95 unlimited, I went with it and cancelled the XM. I often consider resuming the XM in addition to Napster, but between the two Napster is my preference. I'm only interested in music.
XM positions itself as the "music lover's" service. Siri and XM have a lot of overlap in mainstream RnR programming, but XM goes well beyond with live concerts, artist- and label-oriented retrospectives, and a truly amazing music archive. I can't emphasize the music selection enough. Eclectic and deep.
Sound quality is similar, although XM claims theirs is better and Sirius claims Howard Stern and the NFL. Overall sound quality is like a decent mp3 or WMA. I played wav tracks against it and found a LOT bigger difference between a good and bad cd mix than between XM and its cd counterpart.
Both Sirius and XM are putting their radios in rental fleets, so you should thank Sirius. It's true, the difference between am/fm and satellite is dramatic. I will put XM in my next car, but in the meantime I just hook up my ipod and go.