It's really hard to explain this device without actually using it.. it acts as a type of scroll wheel slash controller.
AS an example, in Quciktime, you could use it to scroll thru a movie and have the click setting to turn off and on the video, while the click and rotate changes the volume.
I use mine for mainly scrollint webpages. Instead of using a cumbersome mouse with the scroll wheel, I have my pro mouse on one side, and the PowerMate to scroll down thru the pages, and the click to bookmark the page. I don't use the rotate and click function that much, and the long click is set to open a new windows. So instead of using the arrow keys, and the combinations, I just click the powermate. Also, for Mail.app, have you checked out the combination to check mail, well, I have that programmed right into the PowerMate.
Cheers,
RtC
P.S. If you need more info, feel free...