I've gone through a number of FM transmitters for my iPod. My commute to work goes through the periphery of both the Philadelphia and New York radio stations, so finding a clear frequency to use is generally a challenge.
Some time ago I tried the original iTrip. It worked well enough, but changing channels was a pain. The iTrip came with a set of mp3 files, one for each frequency. The mp3 file had a signal that the iTrip would recognize as a command and change the transmitting frequency. While innovation, it was a pain to do as you would have to stop the song you were listening to, play the "command" mp3 file, and then go back. I ended up returning it.
Since then, I've used the Monster iCarPlay and iCarPlay Plus. While these worked pretty well and had the benefit of charging my iPod at the same time, the limited number of channel selections on the original and the confusing controls on the plus model made it far from optimal.
Griffin, the company behind the iTrip, apparently thought the same thing I did about the iTrip, as they've recently released a new version called the iTrip LCD. It is about the same size as the original iTrip, but has a large metallic dial to change frequencies. A backlit LCD screen on the iTrip displays the selected frequency. It also has a configurable setting for sending out a stronger mono audio signal vs a "standard" stereo signal - great for podcasts. Sound quality is about what I've come to expect from a transmitter - not great, but good enough for most use.