The Iphone automatically connects when you turn the ignition on, there are no battery's to worry about and the signal quality is surprisingly good.
The only down side is that the hands free isnt the best but still works reasonably well.
All in all I think its a cracking low cost wireless option to connect your phone/mp3 to the car.
http://www.belkin.com/uk/null-Belkin/p/P-F8M117