I'm in the market for one as well. There are mixed reviews of the iTrip in a
previous thread, some think it works great. Check out this review of the iTrip at
epinions.com. Not bad.
As for other FM transmitters I haven't looked lately but the reviews for Belkin, iRock, et al, seem at least as mixed if not worse (think I saw some at ipodlounge.com or ipodhacks.com, Applelinks.com...).
A thread at
iPodlounge.com contains some feedback on AudiaX/Gruv-X. However it doesn't rate that high at
epinions.com. In general I hear a lot of people complaining about battery life in all of the models that require them.
I think what I've gotten out of my searching is that if you live somewhere with a high density of radio stations, you may be disappointed with ALL brands of FM transmitters.
As an aside, in the above iTrip review the author describes an interesting hack to get a HUGE boost in reception from the iTrip.
you can dramatically improve the range of the iTrip by simply rubber-banding a length of wire hanger to the top of the iTrip/iPod ... place it on top of the iTrip and wrap a rubber band around the whole package. I got a 700% improvement in reception with this very simple antenna system. The receiver reported a signal strength of 40 db with the wire and only 5 db without (at 15 feet.)
Sounds pretty ugly! Wonder if it really works.