KDE uses Phabricator, or at least did the last time I contributed. Worked pretty well in the collaboration aspect for submitting a change, getting code owners to review and giving feedback. I was able to jump in as a brand new contributor and get a change merged. The kind of change that would have been a PR from a fork in GitHub.

However I got the distinct feeling the whole stack would not fit as well into an enterprise environment, nor would the tooling around it work well for devs on Windows machines that just want to get commits out there. It's a perfect fit for that kind of project but I don't think it would be a great GitHub replacement for an enterprise shop that doesn't have software as it's core business