Hi this is Stuart from Korlon LLC. I decided to write a case study about ACLs since they don't seem to get a lot of attention. If you've ever wondered (or asked) "what if some people should only be able to see certain data in Civi?" then ACLs will probably accomplish what you want.
ACLs link up with your CMS (Drupal or Joomla) to create a permissioning system that, in a nutshell, allows certain roles in your CMS to view or edit certain groups in CiviCRM.
We used ACLs for a client last summer and the results have been positive. United States National Committee for UN Women is the USA national presence for the international UN Women organization. UN Women promotes "gender equality and the empowerment of women." USNC is mainly a membership-based organization. They have many regional Chapters across the USA in addition to a national staff. The Chapters do local fundraising, but membership is national, and each member is assigned to a regional Chapter (if one is available in their area).
The national staff decided that Chapter leaders (called admins) should have the ability to see member data and contribution data - but only where relevant to their own Chapter. Enter ACLs!
After some discussion, these rules were established.
You can read the full case study here.