Mercurial is like a git designed for humans. Basically the same design (distributed, branching and merging is very easy) but with slightly higher-level command line options that removes some of the rough edges. I kind of wish it had won over git.
The obvious counter is that Git is like a dvcs designed for maintainers and only incidentally for developers. I started out using bzr and eventually switched to git, partly to try out github, and things like rewriting history have become a major part of my workflow (I'm picking that one because it seems to surprise people coming from other vcs systems how easy it is to rewrite history in git).
I'm sure mercurial is nice, and given its users passion for it I might recommend it to people starting version control for the first time (I'm in academia, version control is surprisingly rare), BUT... I'd use the term "very sharp edges" and not "rough edges." My impression is that they're meant to be there.
Mercurial is awesome. I never use GitHub, except when I want to download a zip of some code to review. Mercurial I use on any project not in TFS.
I've tried to revisit GitHub many times, but I just don't get it - it's not as good as Mercurial or TFS, and I have no desire to 'follow' anyone or watch a project. I'm just too damn busy building apps and projects to pay any attention to anything I'm not actively working on.
I'm still waiting for the 'ah ha' moment on why so many people are talking up GitHub, but it hasn't happened yet.
