First let me say that I've had XM since 2001 and love it.
Sat Radio is WAY more than NASCAR. 100% Commercial Free Music Channels + talk(including Ohpra for the wife), news, comedy, local traffic and weather, baseball and other sports programming. XM has deeper playlists and less repetition then Sirius.
NASCAR is something you WATCH on TV. Have you ever listened to NASCAR on the radio? It's annoying at best. I can count the number of times I listened to NASCAR on XM on one hand. That's over the coarse of 4.5 years! What I DO frequently listen to are the various NASCAR related talk shows on XM. To me, these are more important then listening to the actual race.
XM recently signed Dale Jr and Jimmie Johnson to host shows. While I am not a fan of either, I think these types of shows will be good and something I'd actually tune into.
As for the comment about not paying for radio. How many of you don't have cable(or sat) tv? People originally said that nobody would pay for tv. They were wrong.
For anyone that's on the fence, just pick up the equipment and sign up. Bestbuy and CC may even have some of the under $20 receivers left. The $12.95 is a small fee to pay for all of the entertainment you'll receive. Heck, I save that much not having to buy CD's anymore.