Git's original tagline was "the stupid content tracker". Git-annex inverts that, by managing files with git, without checking their contents into git. While that may seem paradoxical, it is useful when dealing with files larger than git can currently easily handle, whether due to limitations in memory, checksumming time, or disk space.
I've been working on git-annex for two weeks, and have been trusting my data to it for a week. Not a little data, but terabytes spread amoung multiple archival drives, file servers, laptops, and portable USB drives. In situations too complex for unison or rsync to be of much use. Applying git to this problem in an unconventional way has simplified things enormously.
At this point it should be very usable, and solid. A package will be uploaded to Debian shortly; or git clone it.