Use caution if you have a "Metallic" paint car.
I install public safety radios for a living, a company called UNTENNA makes a low profile similar to the XM unit. A couple were painted to match the metallic paint and suddenly dont work due to RF interference reflecting off the flakes. We had them switch to plain color paint and they work fine.
I dont know if XM will be inhibited because there are Satellites everywhere, but in tunnels and heavy tree'd areas it drops off.
Just a word of caution..