It's very well-designed and has support for enterprise features like ldap, clustered servers, CI, etc. and there's no distinction between community/enterprise versions -- it's all open source. Other great features are task management, wiki, blogs, question/answer (a la StackOverflow), and of course memes/macros which add some lighthearted fun to code review. The engineering behind the product is top-notch which is evident from reviewing the tasks/discussions behind features [1], or even reviewing the source code. One killer feature for my company was Spaces, which allows setting up "walled gardens" -- we use it to allow external collaborators to create tasks, etc. without being able to see anything outside their own space.
Managing and maintaining Phabricator is pretty straightforward. It's a standard LAMP type of product (Linux, Apache, MySQL, PHP) which is easy for myself or IT to jump on and diagnose issues. It's the only tool I've seen which will also alert administrators to identified issues with configuration (when logged into web page). Additionally administrators of the tool do not have blatant all-access to projects, though that might be niche.
I'm glad it's getting more notice now, even if it's largely due to Microsoft acquiring GitHub. I don't know if Facebook uses Phabricator still but WikiMedia does and I belive the Mercurial development team is considering it [2]. Phabricator is a great product and worth trying out. Their community is located at [3] - they used to allow open access to their self-hosted Phabricator instance but it became too noisy as the user base grew so they've moved to discourse forums.
[1] https://secure.phabricator.com