Mercurial is one of the many sad stories of far better technology being forgotten by the popularity contest juggernaut of something else.
I still use mercurial for all my personal project where I don't need to care what anyone else thinks. It is pleasant to use good tools, just like I like to buy top quality rachets or such.
And in a similar vein, Darcs. It unfortunately couldn’t compete with Git on performance, but the user experience is on a whole another level.
Mercurial wasn't the better technology, though. The UX is almost the same as git, diverging in ways that are arguably worse, but the tools were written in much slower Python (initially, and for many years after).
Can't comment on Mercurial, but "for all my personal project where I don't need to care what anyone else thinks" I am using Fossil. Ever since that decision, I've felt a bit, well, held back, or rather, I don't feel quite as comfortable as I do at home when I have to use Git.