aldive, I have the same GPS as you. You don't have problems on a boat because, in the water, you always have a clear view of the sky. And if you're using DGPS, that's great, but I doubt people have DGPS in their car!
My GPS is permanently mounted in my X, with my antenna mounted on the inside of my windshield. In rural, hilly areas, as well as downtown areas with tall buildings, I regularly experience momentary dropouts. This isn't so bad with GPS, because you don't need it every split second, but with XM radio - this means you've lost your music. Also, I wonder if these XM radios are 12-channel receivers? If not, then you'll have a hard time maintaining a steady signal. Considering the DoD spent billions of dollars deploying satellites dedicated to GPS - and THEY deliver 12-channel signals - well, I doubt the XM radio industry spent THAT much!
Just some things to think about. I'd wait at least a year because service & coverage will improve and prices will go down. This always happens with new technology.
Personally, I can't see paying for something I already get for free - FM radio!