Sigil is a free and open-source EPUB editor, that let’s you create a new EPUB file from scratch or edit an existing EPUB file. Sigil is good and bad because of the same thing. It exposes the structure of EPUB file and let’s you edit each part including HTML, CSS and specific files for table of content and metadata information. If you are a technically inclined user who understands and is willing to work with files at such low-level, the tool would work for you. But if you prefer a simpler way of creating e-books without being exposed to technical nittigirtties, you might not like it. One way to use Sigil, in such cases, is not to create EPUB from scratch in the tool, but use it only to make minor changes to the EPUB file you have from some other source.
Apart from exposure of technical goriness, another disadvantage of Sigil is that you can only create EPUB using it and no other format. For creating mobi or PDF files, you will have to resort to some other tools.
Finally, while it is quite a stable tool, currently not a lot of development is happening on it. So, as the EPUB editing becomes more and more sophisticated, Sigil might not be able to meet the needs of users.