Syncthing is great though, which is an OSS clone of Sync. It doesn't offer anything near the features of Sync, requires that you have control over the firewall

Syncthing is great though, which is an OSS clone of Sync. It doesn't offer anything near the features of Sync, requires that you have control over the firewall or the firewall is set up to use uPnP (which is not true for many networks I use on the go), is be difficult to install for friends or family I would want to share files with, and doesn't offer sharing invitation links. Syncthing is great for a subset of use cases of BTSync, it's mostly pre-1.4 BTSync (minus the trackers and relay servers to make it usable when you do not have control over the firewall.). After I saw the awful security review for BT Sync, Well, it was mostly the security review that was awful (did you read it, of course BTSync accesses the update server). iSec Partners also did a review of BT Sync and found it to be secure. Disclaimer: relatively happy user of Bittorrent Sync. Just not too happy with their monetization model, would have preferred purchasing the software, rather than a subscription model.