Looking into the Starmate 5 Dock & Play Radio + PowerConnect™ Vehicle Kit and XMp3i™ Portable Radio with Home Kit.
One I can move from home to any car. I have an fm transmitter for cars without a mp3 slot.
Comments welcome on your experiences with the radios and services.
On our trip to Seattle area last year the rental included one. Worked well traveling around and avoiding changing stations. Even on the Olympic Coast, we didn't get a lot of signal loss.
I'm a big fan of sat radio but not a fan of moving them from car to car.Here is my suggestion
Get a kit to install in the car that you'll use it in the most and leave it there. Buy a subscription to the portable application and use that on your cell phone/blackberry/iphone when in other cars. This obviously only works if you have a data plan, but also allows you to get Sirius/XM in your house on a computer.
If you have an iPhone, then there is a specific cradle for that. That one is a breeze to move from car to car and has a built in FM transmitter.
SiriusXM - Shop | Radios
BTW, don't pay regular price for your subscription. You can usually get it for about $77 - $85 a year if you are persistent ("I don't know if I like it, maybe I'll cancel, It is too expensive" etc.)