In terms of usability and price Sync Folders Pro is far best solution for sync any file anywhere.
Sync Folders Pro is very versatile. It has a lot of customization options. I mainly use it to sync plugins and scripts of some applications between my two Macs on a shared folder. I also use it for backing up my kindle highlights file. Additionally, I use to backup my iTunes Media folder to NAS. There are multiple sync options available, so anyone can find a way to use SyncFolders Pro to fit ones needs.
It keeps a detailed log, so user can check for errors, if any
Sync Folders Pro, v3.3.2