Yes, I've looked at bup. I wish the author would try to get it integrated as part of the git suite (or at least in contrib/). Having to specify things like GIT_DIR before using bup commands seems dangerous. Plenty of git commands keep their own metadata in .git.
I would love to see "git bup!"
The reason I'm sticking with git is that's is easily available anywhere and right now I mostly just use it as a more convenient rsync. That is, I simply want to make a bunch of files easily available anywhere. As a bonus, replication provides a incremental backup capability.