I use it. My work git repo isn't github, so switching between this tool for github and cmd line for work has become part of my habit field. I still go back to cmd line for more complex stuff though.
You can use this tool for any repo, not just GitHub repos. We have a mixture of non-GitHub and GitHub hosted repos that I manage all within GitHub for Mac. When they first released this I was actually using Beanstalk, but still used the GitHub for Mac application to manage those repos as well.
Does it work with svn repos? i.e., if I've git svn cloned a repo on my machine. Can I interact with it with GhfM? Will sync do 'git svn dcommit'?