I've been using my friend Chris' iTrip, and I've got to admit, it's a great little device. I'm definitely going to have to pick one up for myself at some point, along with a cigarette-lighter adapter for those longer trips.
But good thing I don't live in the UK:
But in Britain, using it is akin to setting up your own pirate radio station.
Believe me, friends...you can only call it pirate radio if your definition includes only being able to broadcast about 50 feet, with an extremely finickly transmitter to boot.