Good information in this thread. Similar to another poster, I used Calibre some years ago, got busy with life, and just recently came back to it. I would love to be able to sync basic reader metadata (reading position, context-specific comments, and highlights) on the same book in my Calibre library, then access it from multiple devices: PC, Android phone and tablet, and my new Kobo. But I'm realistic that this isn't happening anytime soon.

One workaround I've been using is manage all the ebooks and keep the source files in Calibre, but output a 'reading copy' of my whole library. That reading copy gets both posted on Google play books (where I can access with any device I have and leave notes, sync reading position, etc.) AND I sideload it to the Kobo for reading only. I dont' bother adding in notes/highlight in Kobo, since it wont sync with anything else.

If somebody has an alternative solution that doesn't involve anymore work than I'm doing now, I'm all ears. :-)