Tapedecks sound better than FM transmitters, but tapedecks are being phased out in vehicles and they're rare in CD players because they're so obselete.
So if you want good sound for a short while, or if you're willing to shell out the cash for a tape deck in your next vehicle, the tapedeck route would probably be the best. If you want OK sound (I've heard that if it isn't a few feet away from the antenna you do notice a difference from the iPod's normal sound, although it isn't a lot) that will last you for a while, go for the iTrip or iRock or whatever.
Good luck!