git-annex is not a backup system. It may be a useful component of an archival system, or a way to deliver files to a backup system. For a backup system that uses git and that git-annex supports storing data in, see bup.
git-annex is not a filesystem or DropBox clone. However, the git-annex assistant is addressing some of the same needs in its own unique ways. (There is also a FUSE filesystem built on top of git-annex, called ShareBox.)
git-annex is not unison, but if you're finding unison's checksumming too slow, or its strict mirroring of everything to both places too limiting, then git-annex could be a useful alternative.
git-annex is more than just a workaround for git scalability limitations that might eventually be fixed by efforts like git-bigfiles. In particular, git-annex's location tracking allows having many repositories with a partial set of files, that are copied around as desired.
git-annex is not some flaky script that was quickly thrown together. I wrote it in Haskell because I wanted it to be solid and to compile down to a binary. And it has a fairly extensive test suite. (Don't be fooled by "make test" only showing a few dozen test cases; each test involves checking dozens to hundreds of assertions.)
git-annex is not git-media, although they both approach the same problem from a similar direction. I only learned of git-media after writing git-annex, but I probably would have still written git-annex instead of using it. Currently, git-media has the advantage of using git smudge filters rather than git-annex's pile of symlinks, and it may be a tighter fit for certain situations. It lacks git-annex's support for widely distributed storage, using only a single backend data store. It also does not support partial checkouts of file contents, like git-annex does.
git-annex is similarly not git-fat, which also uses git smudge filters, and also lacks git-annex' widely distributed storage and partial checkouts.
git-annex is also not boar, although it shares many of its goals and characteristics. Boar implements its own version control system, rather than simply embracing and extending git. And while boar supports distributed clones of a repository, it does not support keeping different files in different clones of the same repository, which git-annex does, and is an important feature for large-scale archiving.
git-annex is not the Mercurial largefiles extension. Although mercurial and git have some of the same problems around large files, and both try to solve them in similar ways (standin files using mostly hashes of the real content).