Source code repo, bug/defect tracking, code-review / auditing, and real time testing. Makes life so much more productive than the 'old' days.
I'm not quite sure what kind of integration with Mercurial you are looking for, but the Mercurial wiki contains one such extension [2].
[1] http://www.reviewboard.org/ [2] http://mercurial.selenic.com/wiki/ReviewboardExtension
See http://mercurial.selenic.com/wiki/TimeBasedReleasePlan
Personally i find it great (and we try to do the coding sprint one/two months before the next release, so the big features have some time to be stabilized).