We use a Kensington Digital FM transmitter/charger and have been very pleased. It's a tad $$ but IMO better than the Griffin or Belkin models. We had a Griffin iTrip and it only worked if you had the transmitter right next to the FM receiver antenna. It also depends on how many FM stations are in your area but we live in the metro Atlanta area and are able to use it with very little interference, especially if we stretch out the cable. On a road trip, I'd think it would work great.
I have been using the kensington as well and have had very few problems with it in the 2 1/2 years that I've had it. Used it with my 3g iPod and now my iPhone. The only down side is that I sometimes get a bit too much sibilance and it really boosts up the bass (so much that I sometimes have to change the EQ settings).
Something else you may want to consider instead of an FM transmitter is a cassette adapter, if you have a cassette player on your car radio. You'll get better quality sound out of those than an FM xmitter.