I had a few problems with my tuner:

• "Now & Next" info mode doesn't work
• Tweak Order doesn't work
• Mark Track doesn't work (I wanted to mark a load of stuff for re-tagging)
• None of the shuffle modes seem to work, except Random

So I've gone back to MP3s.

On the plus side, the compression seems much better (I didn't notice any change in my free disk space), and the passengers seem to prefer the tuner's playlists.