I have used Git, SVN, Mercurial and, yes, Visual SourceSafe because as a consultant I have to use what I'm given a lot of the time. I have used Mercurial for some of my own projects. I'm sorry to see Mercurial's simplicity, usability and sophistication undervalued like this.
Git's blazing performance is hard to argue with. Not having to wait for stuff to happen is a big draw for me as for many other developers. I think that's a good part of its appeal. That and the fact that it is very capable in the right hands. But Git's performance is a confidence trick, and its time may yet come.
I have always dreaded Git merge requests on big projects with many contribultors. I know projects that take days resolving merge conflicts between feature branches and trunk. This is because large feature branches are a Bad Idea that many organisations still don't realise is one. But it is also because Git is based on poor mathematics.
There is a better way than both Git and Mercurial.
I'd like to see Pijul succeed. https://pijul.org/
Pijul is based on Darcs-- https://en.wikipedia.org/wiki/Darcs. Darcs and Pijul are based on a mathematically sound 'patch theory' that significantly improves on Git and Mercurial performance during complex merge scenarios.
Pijul incorporates an improved algorithm over Darcs that promises to fix Darcs' occasional slow responses. But it is still not of production quality.
Anyone wanting a meaty Rust project to work on, and to improve the status quo in DVCS, please head on over :)
(Disclosure: participant and Rust learner)