IMHO if you connect the car mp3 player using proper leads (instead of RF tuning it) and burn all your mp3s (from original cds) using at least 128kb/s bitstream , you shouldn't be able to tell the difference from CD. ... go to 192kb/s and you got CD quality!
I got a Phillips MP3-CD walkman , and the mp3s play as good as a normal CD would sound...
for my next car stereo , I'd definately be inclined towards a MP3 player... though thinking about that, it may be worth getting a CD player/changer that reads/plays mp3 off a CD; that way you got versatility
