The sound quality is actually pretty good but be aware that depending on the head unit (there are quite a few out there now with USB ports) the drive you use has to be formatted correctly and the files arranged on the drive just so, to get them to play.
The quality depends on the bit rate of the mp3. And most HUs come with some sort of program to arrange songs. Also, diffrent HUs can only support limited gig drives. Most common if 2 gig and 4 gig. With out a program, songs will just be transfered the way they are placed onto the drive. Then they will play that way. With an editor program, such as PhatNoise, you can arrange diffrent folders, w/ songs in each.