I'm also a big fan of the FM adapters. I've got an Irock that I picked up from Radio Shack for my jukebox. They're not obnoxious at all, the clarity is good (on occasion you may have to change the frequency or move the transmitter around but for the most part it's crystal clear), and they're very unobtrusive.
With the Irock the only thing I can tell you is make sure you mind the power button. It's VERY easy to turn on accidentally. It's a big button and requires very little travel. When you only get 8-10 hours per battery on them, and it's been on for 4 without you knowing it can make the car trip you planned on listening to your new playlist seem a bit longer.
