I have used this app only once when Amazon was refusing my book in Calibre's mobi format and it saved my life. :-)
I opened with Sigil the epub generated by Calibre and generated the TOC (Tools/Toc/Generate at that time) then I opened the edited epub in Kindle Previewer or kindlegen and converted to mobi.
So 5 stars for this invaluable tool!
8.01 is out at http://sigildev.blogspot.it
A really nice and great application to create or edit epub.
My only complaints with these last versions (8.0 and 8.01) is that you can not paste text in the Replace field using key shortcut anymore.
It is not a tragedy, but it's not so comfortable activate the control-click menu every time you want paste the replace string in Search and Replace…
So before to upgrade take also a backup copy of previous version if it works for you.
Great software, unfortunately no longer under active development. See: http://sigildev.blogspot.com
I am still using it in Mavericks. I use Calibre also, which is constantly improving as an ebook editor.