Here's a nice review of a few iPod transmitters.

-->Link<-- (http://ilounge.com/index.php/reviews/comments/the-fall-2005-fm-transmitter-shootout/)

Bottom line: Kensington's transmitter (http://www.kensington.com/html/6402.html) is the best.

I wouldn't really trust reviews like this. They are all pretty subjective. FM reception varies due to quite a number of factors, including the type of car you're driving, local stations and quality of the source (which, if we're only talking iPods is different).