I've been using BitTorrent Sync for the past week. For the use case of sharing large files, it obliterates Dropbox, AeroFS, and Cubby.
It distributes the files intelligently and makes optimal use of everyone's bandwidth. Dropbox, for example, slowly uploads all files to the cloud before distributing them, plus there's the space issue. AeroFS allows unlimited space, but is far slower than my Internet and LAN speeds allow, and does things like trying to upload the same file, linearly, to every peer at once. Cubby has limited space and has the same slow syncing problems.
I'm running BitTorrent Sync on my 6 year old Windows 7 Thinkpad, a newer Windows 8 desktop, a Digital Ocean Ubuntu VPS, and a Synology DS110j NAS. It runs perfectly on all of the above, and provides a useful web interface for the VPS and NAS.