You can use it, and a few of us have already been doing so. There isn't any Trac integration yet, but it works nicely for patch review. Right. I was wondering about the inclusion of phabricator utilities in the GHC tree - I believe this was mentioned in the discussion.
There's a short intro doc here: https://secure.phabricator.com/book/phabricator/article/differential/, but it's not hard to figure out the basics, and you'll learn by watching how other people use it. No doubt, although I must say that it doesn't yet feel intuitive and some UI things could be done better.
If you go to the Herald tool you have yourself automatically subscribed to diffs that touch areas of the code that you're interested in. Is there a way to create a notification for a particular directory in the GHC source tree? I see I can create a notification for "Any changed filename contains ..." - if I give a part of path (say compiler/basicTypes) will I get notifications if anything in that directory changes?
Pro tip: the keyboard shortcuts are really useful, especially "z". Hit "?" to see all the shortcuts. Duh, no C-x prefix ;-)
Janek