I've been using the iTrip for a while... not too pleased with it, as it has to be in a certain position to work well - aligned in some way or another, can't just "toss" the iPod + iTrip into the central glove box and let it rattle around.
Nothing worse than being in full Attention Seeking Whore mode, windows down, glasses on, Z burblin and howlin, then your tunes playing static instead of the heavy bass! Sucks.