Yeah, being able to get my data back out is a hard requirement for me too. It's one of the things that put me off of OneNote.

A few years ago I wrote a few python scripts to export the Notes.app database to html and bear formats. I haven't really maintained them (although I still run it in a cronjob for backups), but I put them up on github in case it was useful to someone.

https://github.com/dunhamsteve/notesutils