I like the user activity log, but it is very limited.
Basic need: Ability to export the user activity to excel or csv file. This is should done by the user and the administrator for a specific users. Mainly for audit and review changes made by a specific user.
More functional need: Ability to use Aha! list and pivot table reports to against user activity and other objects in Aha! such a release, product, feature, and so on. The value is to be report to audit who, when and what changes were made for a specific product, release and feature.