Excellent program. While some of the options are a bit obscure to find or work with, it will allow you to do nearly anything. The user manual needs serious work, making it a bit more difficult to learn how to do what you need; it's tech friendly but not particularly user-friendly. I still rate it 5 stars because of its wide range of features and options, the ability to edit eBooks directly, and its streaming library capabilities. Needs some user-friendly work but still an OUTSTANDING program.
Calibre requires import (copying) of the users books. If the number is small, Calibre is OK. If the number is large, unless the user has plenty of disk space to duplicate his library, then Calibre creates problems: 1.Where to make space to duplicate the library? 2. After import, shall the user trust Calibre's import in order to delete the original library? The authors do not consider the time a user may have devoted to creating an operating system structure of files and folders, neither do they consider the work done by the user to be useful. This assumption is false. The authors refuse to consider the obvious solution - provide an option to leave the original library intact and use links to the books.
It's a great idea to be able to convert e-books from one format to another. However, some of the key conversions (like epub to mobi) that users would frequently want are very poor and produce a product which looks substantially different from the original and is much harder to use. You have to learn which conversions work and don't work, and adjust what you do occasionally. For example, if you want to convert an epub book to use on Kindle, you would probably want it in mobi format but you should instead convert it to PDF, because that works well and the mobi is very poor.