The Wikimedia project and some other high profile projects use it. I did a pretty thorough evaluation once which included Phrabricator, it came out very well,

The Wikimedia project and some other high profile projects use it. I did a pretty thorough evaluation once which included Phrabricator, it came out very well, especially the work flow around code review. It wasn't chosen for unrelated reasons. I don't know why it isn't more popular than it is. It seems to be very mature, certainly better than a lot of tools that organizations pay good money for.