After using this plugin for several years, I can tell you it has only gotten better, which is amazing because it was already the best of its kind. So, below is my original review, and it is still true to this day.
This is the only library categorization plugin for WordPress I can recommend and I have used some of the more popular ones in the past. It's easy enough for an end-user to understand, which is very important since my clients take full command of their websites. This puts a very special stress on good UI design and your plug-in really impresses in this regard as well.
I really like the level of detail revealed for a media item, and I really like that I can apply categories to multiple media items all at once. The plugin works flawlessly and I can see no conflicts with NextGen Photo Gallery, WP-Property, or others within the iThemes Builder framework, using the "Foundation" child theme. Your plug-in is clean, fast, and intuitive, without sacrificing feature set. Nice job. A donation is on its way!