I use Notebook too. Its sync to Dropbox is very useful (and is a main reason why I use it in preference to OO, although there are others), but is a very partial implementation. If you edit a notebook at either end (Mac or iPad), you have to sync *before* editing at the other end. If you don't, you have to choose which copy to keep. It's less like syncing than like document sharing. I've had issues with the process, although I've now got a workflow that seems to get around them.
Has anyone here looked at CarbonFin? As far as I can tell, it is vastly inferior to OO, but it has two syncing options for its iPad app: Dropbox and its own proprietary service. Both work flawlessly.
CarbonFin also has a feature that would make Omni's proprietary sync server instantly useful: You can publish outlines to anyone, whether or not they have the app. If Omni had a way to publish the excellent dynamic HTML export online, quickly and easily, and keep it synced with the originating outline, that would make the proprietary Omni sync worth paying for.