But it doesn't store your data, so yes, one device must always be on in order to sync with a new device. However, if you have a device that needs changes made on another one, it will just wait around and sync whenever the device with changes comes back online.
For me to consider switching to this service from Dropbox, it would have to be able to temporarily store some of my files from my work computer so that they could be transferred to my home computer later on when that computer was started up (for example). That is the feature that needs to be implemented for this to truly become a Dropbox-killer.
If I have to devise some system to zip my important stuff and keep that up to date, then I may as well just use BTSync for free.
I see two key use cases that could cause trouble:
1)a co-worker turned his computer off and went home from work, the syncing couldn't complete even if only a single doc was left
2)Because there is no central server if all of my coworkers in a distributed team sync with my new shared folder at the same time, it is required that I own a connection capable of supporting that kind of traffic, because it would be almost the same as streaming a video to multiple users, not to mention that I would probably have to avoid to browse the web for pretty much anything else in the meanwhile.
As far as 2 is concerned though, that is not the case. Don't forget, it's built on top of BT. As each piece of the file gets transferred to other clients by the original machine, the other clients will be able to send those pieces out too. In theory it will be faster than a transfer from a central server.
"We also provide such additional methods of ensuring connectivity as relay and tracker servers."