Recently integrated Quill into our app and it is fantastic — as fantastic as a WYSIWYG editor can be, I suppose. It produces valid, sensical HTML as the output, in case you need to take the result and use it in email templates (which was our use case). Highly recommend.
The WYSIWYG editor on Slack that everyone is hating on is actually Quill-based. And do note that it's always been based on Quill, they just recently added more Blots/Embeds to it.
