I just bought my friend an iTrip, and I have to say that it quite frankly sucks. Even on a completely empty channel, the iTrip had static and background hiss. I use the NewerTech RoadTrip! (87.7mhz version) and its much much much much better, even in the same car. The transmitter is much more powerful, and blocks out the background noise completely, plus has almost twice the left/right separation of other transmitters. OWC sells the RoadTrip in both an 87.7mhz and 107.7mhz version for only $20 with $5 shipping... can't beat that price!
They have a sample MP3 on their site that will let you hear what I just described... and its not biased, since they sell all 3 of the transmitters that they're testing 😀iPod Test.mp3
You can check out all their iPod accessories (including tape adapters and FM transmitters) here.