I am using github and I am willing to start using Gerrit with it.
Since Gerrit uses JGit, I am scared of the following text from this link:
EGit/JGit is a reimplementation of git in java, due to this GitHub does not officially support it. Issues have happened and repositories have been corrupted by them. While it is usually a simple matter to repair these repos due to git’s distributed nature, be aware that EGit may not work as well as the official git binaries.
Did someone already use it?
Gerrit is it's own server and the issues we've seen were with the client and have recently been fixed in the brand new version of JGit. It also didn't corrupt the repository, it just pushed packfiles that were missing objects. The Gerrit server is solid and has been used by the Android project for over a year.
However, since it is it's own server and enforces a very specific workflow, it is generally not used in conjunction with GitHub much, since code contributions on GitHub tend to be pull requests, where contributions on Gerrit are peer-verified patch series through the Gerrit system. Again, you can set them both up for the same project, but most people do not.