After my initial fumbling and struggles, I must say that the Sigil Crew have done a masterful job.
The best part is the external interface. Editing the content.opf and toc.ncx have become childs play. No more copy-> paste -> edit -> copy -> paste. Final touchup of images, which always is never seen to be needed before loading to sigil, becomes easy.
And then doing this for mutiple OS is outstanding.
If I still owned my Software Company, I'd hire one and all.