Account Tracker Family Sync
The one main drawback, however, is that iCloud only supports sharing between devices for a single iTunes account (an Apple limitation). This means it doesn’t work if two separate people want to share and sync their data.
Roll on 2014 (albeit August!) and I now have database syncing working over Dropbox. Multiple separate users could therefore use a shared Dropbox account to sync their data.
I will be testing this for a bit myself before releasing it, but you can expect it in a few weeks time.
Note that this will be a separate in-app purchase.