I love it. I use Trello as my 2nd brain but it means that I can't do anything offline. Where I can see loving this is if I write a little converter so I can take a Trello board JSON export, load it into Nullboard and work on it offline, then a thing that goes back the other way and creates a Trello board from my NBX file.
Maybe if I put the original trello card ID at the bottom of each NBX "note" and then synched any text back as a new comment on that card, and the list ID in the title of each list and adding any notes without a Trello card link as new cards to that list, it would be a pretty automated way to get a bunch of edits back into Trello where I could tidy up with copy/paste.
Rock on!! Forked the repo and have my new local version pinned.