Let me kick off this discussion by warning you that FM transmitters -- every one of them, I've tested -- **SUCK.**
The worst, cheapest cassette adapter was able to produce better sound than any FM transmitter we tried, and we (by which I mean our Mac dealer store in Florida) tried all the major brands currently available. They SUCK and should only be used if there is literally no other option.
Consequently, I'm not going to say "well this one sucks least" because they all sucked. But if you really must buy one, get one that allows you to set any available frequency rather than one that's locked to 88.3FM or something.
For myself, after researching this thoroughly, I opted to replace my car stereo head unit with one that had a rear input that could be extended around to the driver side (some radios even have front mounted AUX input, and that's your best choice). WOW what a difference. Yes that option cost more, but the music was worth it.
Both Belkin and Griffin have FM Transmitters for iPods that charge as they play. If your car has a tape deck, you can also go old-school and get a tape adapter because the quality of sound will be better and more consistent, especially if you're in a big city with lots of radio signals flying around.
According to all known laws of aviation, there is no way that a bee should be able to fly. Its wings are too small to get its fat little body off the ground. The bee, of course, flies anyway. Because bees dont care what humans think is impossible...I want to be like a bee.
Invest in a better car stereo that has either a 1/8" stereo aux/input jack on the front, or a higher end model that has iPod integration. You will be significantly happier than trying to live with an FM transmitter.