Besides artwork, I heard it finds tags for untagged music? Does it canonize tags (e.g. David Bowie, Bowie, David and Bowie end up tagged in te same category)? Does it reorganize folders or just tags? Rename files? Does it suppress duplicates? Does it handle non-canonic tags (e.g. conductor, composer, disc number, etc.? Can you set a format preference (e.g. if you have AIFF and MP3 versions of the same music, keep only AIFF)?
Those are some of the things my ideal library manager would do. Ideally it would also sync it between different discs and have a cloud backup.