The NOOK is darned big to use as a casual MP3 player. It's not something you can stick in your pocket or hang from a neck lanyard.
The NOOK only plays MP3 and OGG Vorbis formats.
There is no ability to organize your music. There are no playlists, not even albums. Just one giant list of every track that you have loaded. The songs appear on the list in whatever order the NOOK wants them to appear in; there's no sorting option.
The headphone jack is located on the bottom. This limits the ways that the NOOK can be positioned while listening to it.
You can play music while reading.
Sound quality on all MP3 players is about the same. There's nothing especially good or bad about the NOOK's sound quality.