This is the only library categorization plugin for WordPress 3.4.2 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!