I can't speak for "users", but fossil is an absolute joy to use, easy to pick up, and has features that I'm not aware of in any other SCM (hello, built-in first

I can't speak for "users", but fossil is an absolute joy to use, easy to pick up, and has features that I'm not aware of in any other SCM (hello, built-in first-class tickets). libgit2 has been "making progress" for a long time -- it's barely used in github (it's one or two little features of libgit that are harnessed internally, according to the githubber I talked to at a meetup), and, without trying to insult the developers/development, I seems more accurate to say libgit is "plodding along slowly". With "critical mass" as one of gits features, this is sort of surprising. I suspect it speaks to the complexity of git, but I'm not positive.