The Glasgow Haskell Compiler, also known as GHC, is moving to GitHub after contemplating other version control systems. The choice of version control system was chosen after an analysis of other version control systems:
It came down to two things: the degree of support available, and flexibility of the tools (git is much happier to let you modify the history than Mercurial). Speed ruled out bzr, and Windows support is less of an issue: git appears to work reasonably well on Windows these days.
This is notable as GHC used to use Darcs, a distributed version control system written in Haskell. A comparison of the files in Git and Haskell showed the distribution was centred towards C files for Git and Haskell files for Darcs.
The two leading distributed version control systems are Git and Mercurial, with Darcs, Bzr and others much less widely used. Typically the systems are used by their language implementers; Darcs, by Haskell developers, Mercurial by Python developers and Git for C developers.
What this means for Darcs as a version control system remains to be seen. Entries on Planet Darcs are still being updated, but there are less users now. Performance appears to be one of the significant reasons; but popularity also played a part, with the GitHub clone now hosting 4 forks and 72 watchers.
The Haskell Darcs repositories were locked for migration yesterday, with the migration to Git happening subsequently.