I had XM installed in my IS 350 last week. Receiver and antenna have to be added and it isn't cheap. I didn't want an non-matched antenna on my trunk lid, the dealer (Lexus of Bellevue) suggested placing the antenna under the deck, completely out of sight. So far it is ok but if I am in area where the signal is weak I lose reception earlier than with an externally exposed antenna. (I used to have a 4Runner with an external XM antenna so I know where the signal is weak on my regular routes.) If a color matched shark fin becomes availalble I might go for it.
Receiver cost was $250, installation kit was $140. Add labor and you have one expensive radio. Monthly subscription is $12.95/month for XM. I tried to live without XM for a few weeks but once you have tried it, it is hard to go back. Kind of like buying a Lexus.