I had XM in my vehicle for a little over a year and did listen to it whenever I would travel. On my AM / PM commutes I would listen to NPR on local FM station, but XM was awesome when traveling.
But after around a year, it went out on me...display just read "ANTENNAE", so I assumed there was a wiring problem. 1 year warranty from Circuit City, who installed it, was up so I didn't rush to have it repaired. I started to listen to my iPod on the road and found that I really didn't miss the satellite radio that much. After a couple of months of paying without getting it fixed, I just canceled the service (which was somewhat of a pain...think AOL) and strictly go with the iPod now.
Between the two, XM was great with an extensive lineup, but I think that Sirius offered more sports, IIR.