I use a Motorola T505 in conjunction with my Crossfire Roadster's Nav Radio. When matched to the iPhone and signal sent to the FM channel chosen by the T505 - it works very well I've been told. Too, I think my end sounds just fine... all of that at highway speeds with the top down.

The only issue is that it is a bit of a nuisance if you are listenting to a DVD and a call comes in... fumble with the iPhone yelling out, "Hang on a minute!!!" while you change over to FM and activate the FM transmitter and then press the Bluetooth option on the iPhone. By then 30 seconds have gone by and you just frustrated everyone. I have the unit on its clip on the driver's sunvisor.

That said, if you are already to receive a call and have nothing else playing OR if you have placed the call - then it is perfect. Alot less than buying a new head unit ...

NEW Motorola MotoROKR T505 Bluetooth Car Kit - Iphone 4 - 16/32GB | eBay