Sunday's Washington Post reviews several options for using your iPod or other MP3 player in your car. They review two FM transmitters (the iTrip and the Belkin TuneCast II) and three cassette deck adapters. The reviewer prefers the iTrip to the TuneCast and doesn't have a problem with any of the cassette adapters. He does note that most cars don't have cassette decks anymore and that neither one is superior to the simplest (and least common) option: a line-in jack on the front of the car stereo.
Last weekend, on our drive from Portland to Seattle and back, we used a TuneCast II to listen to several podcasts and albums on my iRiver H140 through the car stereo. The TuneCast did its job as designed, but suffered from the same problems that the reviewer points out: it sounded tinny and distant most of the time, like any other FM radio station, and we had to adjust the frequency on the TuneCast every hour or so, as we moved in and out of the range of various FM stations. It was better for rock and blues music than for classical, as we found out during a brief attempt to listen to Beethoven. The audio sounded slightly better when I placed the TuneCast next to the gearshift, but maybe I was imagining a difference. I admit that it's an imperfect solution to the problem of using an MP3 player in a car without a direct connection to the car stereo. On our drive to DC for Easter weekend, I used James' cassette deck adapter with the iRiver and the sound was just as good as the CDs he brought along. But for $30 I was happy with the results.
If you're going to buy a TuneCast, get the "II" version, as it allows you to transmit on any FM frequency between 87.5 and 108.1 MHz. Earlier versions used four preset FM frequencies, and if there was a stronger radio station on those frequencies, you were out of luck. At least with the TuneCast II you have a chance to avoid any radio stations in your area by choosing an empty frequency. Also, get a fresh set of AAA batteries before you hit the road, as the TuneCast only lasts about three hours on one battery. For $20 extra you can get an in-car power adapter for the TuneCast that lets you run the transmitter off the car's power. I don't know why Belkin packages it separately from the TuneCast, except that it can also be used to charge your iPod instead, so maybe they're aiming the charger at a different user group.