[Perkin]

Calibre is great at what it does and is useful for 'personal' ebup creation, it's just not as good as Sigil purely because of what it (calibre) actually does - and how it does it (to get from the html to epub).

@Katsunami, one small example to show some problems with having sigil in calibre.

We ave a nice well coded html with all it's resources (css/images/fonts)
(We're currently happy with formatting but later we're going to decide to change some)

Create two epubs - one in sigil one in calibre -->

Sigil ->
Import that html into Sigil add the fonts (as sigil doesn't include them on import),
add chapter splitmarks with a regex -> split on markers
run the toc-generator
add metadata
check validity
save epub.

calibre ->
add html file to book entry (calibre then rar's all the resources together)
convert to epub
(with the options of split at headers/ toc etc.)

Right, (for arguments sake) we'll assume that they are identical when displayed in e-reader

Now (some time later)...
We decide some of the formatting needs changing and several places throughout the book need a few of the paragraphs tweaked so they match some others.
Try editing the epub and see which you'd rather use to edit.

I think if sigil were added to calibre, a 'workaround' would need to be added (ONLY) for html imports, in which calibre did'nt run it's usual convert process, just did the split/toc/metadata stuff and left the html/css pretty much 'as is'.