Well, right now we "are transitioning." We're still exploring the possibilities.
Our main concern with using gihub.io is that pretty much all of our artifacts are binary. (compiled jars) the largest currently is ~1mb, and are not downloaded often. Binary in git is an odd combination, though one team member is advocating exactly that. We're also considering extending the plugin manager to be an asset manager, which means stuff in it own native format.
We went to GitHub for the source because we wanted to shift to git, (works better fit the way the team interacts... We each tend to take our time on projects, and trying to coordinate without distributed branching was making things take forever) and thought the exposure on github would attract others. We're thinking that we will enjoy the very easy integration with, E.G. Travis.ci (you can do similar things on SF, but need your own build server)