My 2 cents: if you want to go cheap, you can't get a built-in charger. Also, say you want to listen to music at home, I believe most of the chargers need to be used in the car. Tape is probably the way to go in terms of quality, though my only concern was whether or not in-car tape decks might be phased out in the future, though that is probably a good ways away. Now, on to FM transmitters.
Probably the 2 main ones are the Air Play and the iTrip. In all the reviews I read, the Air Play came out on top in terms of quality. I can say from experience that it's much easier to use, given that it has its own frequency changer, as opposed to the iTrip's somewhat annoying, albeit clever, iPod based changer. So, I bought the Air Play. Well, it worked extremely well in 2 of the cars I tried it in (an old Toyota station wagon and an old Acura). However, it also sucked ass in 2 other cars (an old Altima (my car) and a new Passat). So, my advice: buy it from a local Apple store, even if it's a couple bucks more. Test it out, see if you like it, and return it if you don't.
