ack has been an indispensable tool in my workflow for a few years now. I use it several times a day to search code bases... most of the time, not even taking advantage of regexes or all of its many abilities. Just a basic keyword search is usually enough; the output looks great.
The feature I've wanted the most is the local project-specific .ackrc ability. I have some projects where I need to be able to ignore specific files/dirs; I think this feature will address that need.