I had XM (and an mp3 capable cd player) in my old car and my 4x4. Didn't get it in my new beater car as I commute by motorcycle now.
My dad took the add-in XM widget out of my 4x4 and put it in his truck. He loves it and can't imagine getting rid of it.
I won't bother putting it in the boxster... would rather listen to the engine.
