Calibre may not be pretty but for a CRUD app it is usable. People who have complaints about Calibre generally have issues with its chrome and dated layout but nobody dislikes it because it is hard to use. A default GTK style UI always looks terrible on Windows and Mac, both of which have much higher requirements for user interface defaults. This is especially so with Amazon and iBooks setting differing expectations for how an eReader should look like. The issue with GIMP is with both its UI and its UX. Trying to get work done in GIMP is difficult and challenging. It is not a mere CRUD app like Calibre.