I've been using Unison successfully on critical documents for over 5 years.

I've just started using Unison and it exactly fits my needs.

Unlike dropbox, it only does work when I run it, and I can manually sort through the conflicts when I choose to run it (which is a good thing).

It also doesn't require a central server, so I can just use it to sync my laptop and PC.

Tarsnap is used for offsite backup, which I trust much more than myself or dropbox.

Unison has mostly been trouble for me. I'm definitely in need of a replacement for it. Doing point to multipoint syncing, it tends to get confused and need to be manually resynchronized; and above a certain total directory byte size, which I've long since exceeded, it can't sync at all (and you have to manually break your directory into pieces and sync them separately.)