I had one, and was not impressed. It worked sort-of when out in rural parts, but was useless anywhere near London because there were no "gaps" in the selection of channels it would use. It was also illegal, although I doubt the authorities would ever know you were using it (or care). The sound quality was pretty dire, too.
I now have a plain audio cable that plugs straight into the car radio (via a socket in the glovebox) and that's far, far better and a lot cheaper.
My daughter got one of those interface thingies that go into the cassette player - that works well in her car. Mine doesn't have a cassette player anyway.