I've only used a radio transmitter, which works quite well. But no matter which you buy, don't go with the cheapest kind- cheap products will work cheaply in this case. With a radio transmitter, that's disastrous, especially if you live in an area like NYC, which is just stuffed with radio stations. A good transmitter will just have the occassional static.
iTrip is really great as long as you're not travelling through a SERIOUSLY metro area or an area with an overabundance of radio stations like Nashville or New York. Everywhere else, it works great.
Yup. I've found that universally ALL cassette adaptors have poor transference. Even when there is a "line out" option other than just the headphone jack. The equalization is just screwed when you use them.