I have a portable MyFi for XM. I liked the idea I can take it with me (it has a record feature to listen to playback)

Because it is portable I have been able to use it with multiple cars. Yes it uses FM transmition to the stock radio, and the reception can be poor at times. In all honesty some of the rental cars I have used with the sat radion built in have also had reception and sound issues due to atmospherics.

At some point I will probably trade out the head unit (already replaced the NAV part with a Tom Tom that is far more up to date, has North America and Europe maps (I am going to take it with me on an extended vacation in Italy and France this summer).