Have it...love it...works great. Right now antenna is the Terk magnetic type that is up on the roof. I'm going to move it to the top of my snorkel. However, I would recommend the window mount type as the reception (I've been told) is better.
XM radio is great for those long drives. What kind of head unit did you get. I have the old Sony one I just removed the ashtray and mounted it in the opening fits pretty snugly as far as the antenna I just leave it on the dash and take it out when I don't need it