I've used it in the past, and it is quite decent.
It appears to be largely equivalent to the Calibre editor.
Reminds me of a similar looking UI contraption I saw recently: https://staticdelivery.nexusmods.com/mods/1704/images/921/92... (the multicolored database icons at the top)
It's kinda funny how we will spend years collaborating on some project yet not think to spend 30 seconds checking a screenshot into the readme for a project that we supposedly want others to find and use. It's like self-sabotage.
Then you finally feel like you're done but now there's 100 different things that you have to document and you get overwhelmed.
It was started in 2009, and for some reason.. no screenshots.
For me, that breaks trust.
Neither of these are interesting for the (presumptive) target audience of "people who want to edit epub files".
https://duckduckgo.com/?q=sigil+ebook+screenshot&ia=images&i...