I did a project on integrating Mercurial with Mongo before, and I have to say being a Python developer I didn't have too much difficulty in figuring the right API, although the documentation is not so great. The code itself uses a lot of abbreviations which you have to really be patient to get familiar. The Mercurial model [1] is a pleasant read for a couple train ride (read it a few times I encourage). Others have done amazing things with hg too, like making hg available in Javascript (now an abandon project).

Mercurial is actually very easy to use. However, it didn't have the exact lightweight concept of git branch until they implemented bookmark a few years ago. I don't remember the limitation of bookmarks now it has been a few years since I used Mercurial...

I too thank Matt and core developers working on Mercurial. One thing I feel Matt and developers need to address is hosting hg on PyPI infrastructure instead of redirecting to their own server. From time to time the server wouldn't deliver anything and it was frustrating to see our builds fail (of course some say you should have cached the build and not hitting Mercurial server every day).

[1]: https://www.mercurial-scm.org/wiki/Presentations?action=Atta...