The only issue I have with my Griffin iTrip has to do with using it on a trip. As you move out of your "home base" area where you found an open FM frequency, you might come across a radio station on the same frequency. Eventually, the nnn KW station will win out over your iPod's 15 mW radio transmitter. With the cassette adapter, this isn't an issue.
2nd best, of course, is the mini-stereo Aux jack on the radio itself. The best interface is the actual iPod Dock connector that you can get with Audi, BMW, VW, Honda, etc., or other after-market radios.