Syncthing looks similar, and LAN sync'ing is a killer feature for those of us in offices with poor bandwidth.
I actually just dropped Dropbox for btsync the other week and are very happy, it's iOS app even serves other clients with files over LAN.
Btsync however uses quite crappy crypto and is closed source etc etc.
Syncthing mentions they use TLS encryption, hopefully i can roll my own 8k keys.
I tried Sparkleshare last year aswell, but my experience was mixed. Also it relies on quite a different architecture (git) than a "versioning bittorrent protocol".
Looking forward to evaluating syncthing when time permits.
I have seen it working.
The last few times I've set up a new machine locally I've used Dropbox to pull over stuff. It usually moves at 40-50 MB/s (megabytes) which is certainly using my gigabit LAN.
If this is accurate, then I guess this is the issue i have noticed.
1: https://superuser.com/questions/663600/set-dropbox-to-only-u...
UPDATE:
Confirmed by [2], that Dropbox LAN sync indeed only works once the file reached their servers.
"Well, when you add a file to your computer's Dropbox, the file is then synced with Dropbox servers. Dropbox will then initiate the syncing process as soon as it determines a change has been made to the file. All linked computers and shared folders will then download any new version of the file. With LAN syncing, Dropbox will look for the new file on your Local Area Network first, bypassing the need to download the file from Dropbox servers, thus speeding up the syncing process considerably."
It is enabled by default and works beautifully.