I personally do not like the auto mode, and run the full mode all the time. In auto mode and cruise control, truck will gain about 4 mph before the exhaust brake engages. In full mode I control exhaust braking with my right foot, on or off the throttle pedal. I tow pretty heavy for a SRW (3500) at a combined weight of 24,500 pounds. Descending a mountain grade I manually select the gear I what be control my desired speed via the EB.
As noted, use of it all the time saves service brakes. Just be aware that the person behind you does not seen brake lights.
I believe that certain high trim level Ford's with adaptive cruise control use the service brakes in conjunction with the EB when in the auto mode.
Chris