I've been nudged by nice people to post this here - honest !! http://paulhammant.com/2014/02/03/facebook-scaling-mercurial-for-trunk-based-development - (scroll down half way).
I'm a user of Subversion from about 0.7 onwards (via Codehaus). I was at the 1.0 party in Brisbane, CA many tears ago. Like Perforce, Svn is still out in front *for a couple of features*. One that is fascinating for me is composite checkout (sparse directories for you, client spec for Perforce). Google uses the hell out of that<http://paulhammant.com/2014/01/06/googlers-subset-their-trunk/>, but Facebook don't. I compared the two companies a few weeks ago<http://paulhammant.com/2014/01/08/googles-vs-facebooks-trunk-based-development/>. Mercurial does not have a sparse-direcories feature that you could use to simulate a composite checkout. Not does it have, by default, fine grained permission on directories (incl branches). Thus, if y'all were to consider a merger of sorts, Mercurial does not yet have a superset of Svn features. Discuss :) Regards, - Paul.
