Line-In beats Fm transmitter hands down.
I have the iTrip, Belkin FM transmitter and a Sony tape adapter. Of those 3 the tape adapter was the best but there was still a tape hiss, but it was better then the distant sound of the transmitter, plus the fact that there are so many stations in the city you could never get a clear signal. so, line-in was the way to go. Stereo quality sound, just perfect but it came at a hefty price so it is a major consideration because you jump from your $50 and under options to the $300+ options.