It has some serious limitations compared to Bittorrent Sync. E.g. BTSync can relies on a tracker to find peers and uses relay servers in case port forwarding cannot be set up. Syncthing does not have anything like that yet, meaning that it cannot traverse many firewalls that you don't have under control. E.g. at work, all incoming connections are blocked unless they are initiated from within. So, Syncthing wouldn't work. It's pretty much the same with many hotels, etc. So, if you are on the go, it may not be any good.

Another problem that I have with Syncthing is usability. With BTSync I can be pretty sure that if I send a sharing link to family, that they will be able to install the software and use the link. Syncthing would be too difficult to set up for most family members or friends.

So, if you are looking for syncing some machines on a network under your own control, Syncthing is great. If you are looking for a Dropbox replacement that works everywhere with good sharing capabilities, BTSync is clearly the winner of the two.

I've been using it for about 6 months and I've been very satisfied.