Hello,
I have used it a lot of times with my N78 and its usability depends a lot on the car you are using it. Since the car is rounded with metal and the antenna is outside of the car it works perfect if you put the phone outside of the car but once you are inside it depens on how shielded the car's radio is. I have seen it work perfect in some cars and couldn't use it at all in others.
Another downside of it is that if you are travelling long distances while using it its difficult to find an unused frequency because the stations use diferent frequencies in different cities (at least in my country, Spain) so an unused frequency in one city or region may be in use in another and you have to change the frequency the phone is broadcasting in.

Bye.