I’ve been using BTSync since it was released as an Alpha. Overall I am very happy with its’ performance and abilities, I can sync what I want, where I want. I’m not locked in to a particular folder to sync to and from. I did notice that my folders, over time, were taking up more space than they should. Here is what I found. Anytime you delete something from a synced folder on the destination machine (the computer that did not originate the sync) that data is moved to an invisible folder called .SyncTrash on the source machine (the computer that originated the sync of data). This is a nice safe guard, but in the long run it can bloat quite fast. And I do not see an option to empty this folder from BTSync’s menu. So for now I have on a regular basis will manually empty it. Depending on your OS, you have multiple ways and programs to show invisible folders.
UPDATE 07-17-2013: Today version 1.1.42 (a beta release) was available for download. Among other fixes and updates, such as Android support, the .SyncTrash folder is now called .SyncArchive.