I just tested Sigil and it looks indeed very good. Since in an early development stage, I think I'm not going to go into a very detailed crash test yet

but I already have some comments (which I may post as bugs/enhancements later):
My earlier question about it being "too smart" was motivated by my preferred workflow. I like to edit the XHTML+CSS in a text editor (vim) and then I'd probably appreciate a tool to generate metadata and TOC, so I'd wish Sigil not to alter the code if not necessary.
I opened an ePUB with Sigil and it looks like it makes it a single long text flow. Adding a view for separate files would be fine.
A CSS editor would be welcome too. And validators.
It looks like the TOC editor is dependent on HTML tags, that's fine for a start, but it should allow more flexibility, like adding arbitrary TOC entries and directly editing the TOC tree (the depth of each entry).
Is there a way to add linear="no" to spine items?
In the metadata editor, it should be possible to specify whether Author, Illustrator, etc. are "creator" or "contributor", and the file-as property. I see it includes date of creation, date of modification, etc. probably other user-configurable events would be useful too (and set date of modification automatically upon saving the file?)
I guess you already know all of this
As others have said, great work! Keep on with it.