For what I have seen, unison works in a similar way to rsync, although the home page claims it has a few advantages over other file synchronization tools.
Although some of the points are a bit esoteric, I do like the idea that unison is “fault-resistant” in cases of dropped connections or power failures. Not to point fingers, but I have had problems in the past where cut connections left folders with garbled temporary files everywhere.
I also see that unison remains at user level, which suggests to me that it doesn’t need special daemons or kernel modules to do its job. I can imagine a few situations, like shared systems or some network arrangements where it would be nice not to rely on elevated privileges or specific system daemons to make backups or mirror data.
It’s going to take me a while to learn the ins and outs of unison, and it’s unlikely that it will dethrone rsync for me. I’m a little stuck in my ways when it comes to my own rudimentary backup plans.
On the other hand, should the opportunity or need arise, I’ll keep it in mind as an alternative.