Back in the day I was doing some GCC development while upstream was still using SVN. At that time there was both Hg and Git mirrors, and while the GCC repo isn't that huge in the grand scheme of things, it's big enough that the "standard" hg workflow of having a separate checked out tree for every local branch you're working on was definitely painful. While I had been using hg for some personal projects, this was really the killer feature that drove me to git.