Good questions. I think that at the lowest level, the per document model is a nice abstraction to build on top of. But I have a lot of plans on the cloud side to make it easier to integrate into the rest of your backend and your existing dbs.

I also am thinking about adding partial replication to the SQLSync model which could enable storing multiple documents in the same database on the server side.

Lots of different ways to go with this in the future :)