I have to agree with Espresso. We have XM antennas on the dash of both of our Cruisers and reception is good. My wife has a Roady2 and I have an Alpine head unit with built-in XM. I love that thing to death.
Incidentally, if you haven't found it yet, XM Fan is a great resource for install info and a lot of other neat stuff. See the link below.
http://www.xmfan.com