What don't you like about the cassette adapter? The iTrip's sound quality will be less than a cassette adapter. The only issue I have with the cassette adapter is the whirring noise it makes since it's actually spinning the tape.
I regularly use a cassette adapter in my car...even went so far as to remove my cd deck and put the stock cassette stereo back in just to use it with my iPod.
When I travel and rent a car, I borrow a friend's iTrip to use since most rental cars only have cd stereos...it works okay but not great and it's a pain to have to switch the frequencies on the iTrip...especially when on a road trip and driving through big cities.
if your tape deck or cd deck is not stock, take a look and see if there is a line in jack or mp3 jack. My JVC cd deck has a line in jack that I just plug my ipod directly into it. It works great and the sound is very good as well.