I've had a couple of other FM transmitters, one being a very small battery unit, the other a plug-in holder, charger and transmitter combined. The first is neat and portable, but battery life poor. The second is neat and holds the Ipod at a good angle for viewing, depending on your lighter socket location. However, both of these offer only a few preset frequencies, meaning that interference from other stations is likely.
So I bought the Griffin iTrip to overcome this. Firstly, the ability to select frequencies is great. I haven't yet had any problem with channel interference, although to be honest I haven't been on a long journey with it. The charge facility is good.
My gripe, however, concerns the output volume. Even with the Ipod volume turned right up, the output is low, which means that I have to have the radio turned right up. This means it is susceptible to picking up intereference from the car. OK, the sound is fine for most stuff, but to turn up so that kids can hear in the back it's not so good.
Also, the trailing lead is a little messy.
Best solution will be to for a wired solution like the dension Icelink, but this is not cheap and not portable.