I cannot comment too much myself, since I only tried it for smaller projects, and nowhere near the amount I gave to hg/git/svn/and others.
That being said, it's not any different from hg/git in terms of cooperation, and more similar to hg in the sense that history is irreversible by default.
But the UI (cli and web) is very well designed. There's nothing to dislike, really. I couldn't care too much about the wiki (IMHO, a better solution is use md documents directly in the source), but the issue tracker solves distributed bug tracking in a very elegant way. Contrast to git/hg, where you have to use third-party or centralized bug tracking with poor integration.
Fossil is used by the sqlite project (and several other hidden gems).
I still find unbelivable that fossil has such a well-designed commit graph and interface, and git (with so many eyeballs looking into it) has nothing similar (and I _do_ know "git show-branch").