Disclaimer - I'm completely and utterly impartial as Mu-An works with us but Pivotal Tweaker (or Mu-Tweaker as we refer to it internally) rocks.

Pivotal was hard to scale as we ramped up stories because we couldn't get an idea of the state of play for any individual developer (i.e. how many stories are in icebox / backlog / current).

Pivotal-Tweaker lets you just see the stories for a particular person and see how their workload is distributed. Which is really useful. Thank you Mu-An :)