I got one a couple years ago, but it was just awful. Here is a link to a similar version:
http://www.tangshop.com/wirfmunhanca.html
Problems?
1.) The microphone clamped over the cell phone speaker hole. The quality of the sound it picked up was very poor.
2.) The contraption stuck into the cigarette lighter with several adjusting knobs that did not stay in the correct position.
3.) It was difficult to connect and disconnect.
4.) There was feedback from the car stereo back into the cell phone.
Best solution in my opinion?
Spring for the Bluetooth ear piece. It's handsfree and a lot more convenient.