Does it work properly for recorded books
Does the player restart at the same place where it stopped (useful for recorded books) or does it restart at the beginning of the "song" (totally worthless for long segment recorded books)?
Many MP3 players restart at the beginning of a segment -- OK if the segment is 5 minutes long (one can relisten to part of a book) but totally worthless if the segment is several hours long.
Some only restart with an index (start of a playlist) -- totally worthless.