I would avoid going the fm transmitter route at all costs. The bottom line is a few years ago, the FCC really clamped down on the signal strength they could put out which basically made them useless. I have an older one that I bought for my first ipod about 5 years ago and it is like night and day as far as static, ability to pull in alternate stations and competing with other frequencies being used, etc, but it's not compatible with any iphones. I bought a new Griffin and didn't have any luck with it. As a matter of fact, the guy at Best Buy told me that they get a ton of returns on these types of products for that very reason.
If you don't have an aux input or a newer car with bluetooth, just go the wired cassette into the headphone jack of your iphone. I know it doesn't look as cool or charge your iphone, but believe me, you'll save yourself lot of aggravation.