I had the Satechi FM transmitter. I needed the extension to fit my 12 volt receptacle. It didn't hold the iPhone well, so I returned it.
I now have the Satechi FM transmitter with BlueTooth. I'm satisfied with it, but FM transmitters, in general, don't work well everywhere (geographically) & may require FM frequency changes.
I can recommend the griffin iTrip, just make sure it's the newer version, 3gs compatible. Perfect, especially if you'd like it to charge your phone and not need a battery. It used to cost a lot IMO, should cost a bit less now.