We run a system where we have a lot of data on a server that we want to see locally. Edits are made both locally and on the server. At the moment we're using Aerofs to handle the syncing. As a general rule it works really well. The issue we've run into is with pre-seeding new machines (ie, when our spot instance based server get killed). I need to play around a bit more but it's time consuming to get a machine back online and totally synced (there may be a solution to this).
Mostly it's a pain being at the mercy of someone else's magic. Aero is great, except when it isn't - and we don't know the rules. That's a real pain if it's business critical. BTSync looked great - until I realised that it's not opensource either.
There are a few syncing options on there but they don't all tick all the boxes. Unison doesn't trigger on fs events so you have to run it manually. BTSync isn't opensource. Dropbox syncs to their servers and back. With AeroFS you don't always know how it's doing what it's doing.
So far AeroFS has been the winner for us. BTSync definitely looks like it fills the same space though.