Thanks for sharing. I recently launched into writing an ebook (educational, kids). Even though I use Word for tons of stuff I became concerned that it would insert all sorts of unwanted junk into the file. I looked around and decided to start the process on Sigil. My reasoning was that I could easily move the text into just about any other platform if I wanted to.
My first impulse was to simply write it all in a plain text editor and deal with formatting and producing all the various file types later on. One file per chapter, etc. However, with Sigil I can deal with images and TOC from the very start, which might be an advantage.
It's interesting to read about how other's have approached this. It sounds like the toughest part of the job might very well be getting the various formats to look the way they should.
If you are doing ebook only (no pdf) and it has limited formatting it should be pretty simple to get it to work on most devices.
Though I've yet to actually publish physical books (only have proofs), I'm not willing to commit to using HTML as the base format. (Sounds like princexml might help though if you decide you want to go the physical route later).