The only people who are unhappy with Mercurial are git users. Everyone I know who is coming from something else to Mercurial never have a desire to leave it.
It has a lot (most, really) of the power of Git, but no one has found a need to make an "Oh Shit Hg" site, because it's not trivial to screw up in it.
Because it has a separation between simple and advanced features (many of which are disabled unless you explicitly enable them), you don't get people wasting a lot of time trying to find the ideal workflow in Mercurial. In a sense it's a bit like Perl vs Python. Perl has many ways to do the same thing. Python is more opinionated, and has preferred ways of doing things. As a result, a team of N Mercurial users will usually not have N different workflows. In my current team, there's been a lot of thrash/arguments over the workflow (rebase vs merge, squash vs not, linear vs branched history, history rewriting vs not, etc).
The documentation, though, sucks. It's quite outdated.