If you live in the mountains and haul heavy cargo, an exhaust brake is a great safety feature. During my cross country trip, the exhaust brake let me drive 45-50 mph for miles on end on 5-7% down grades in the Rocky Mountains, with very litle use of the service brakes. My total weight with trailer was about 31,000 lb at the time.
Alternately you can drive at a slower speed and rely on the inherent braking ability of the engine and drive train alone. In any case, the exhaust brake spares the service brakes for emergencies and quick stops by not overheating them.
My exhaust brake is activated by air. A butterfly valve closes the exhaust pipe to provide resistance during the exhaust stroke. Works best at high rpm since the effect is directly proportional to the number of exhaust strokes, duuh.
The back pressure is about 35 psi.
Very few truckers would attempt a down hill run without an engine brake, be it Jacobs, Williams exhaust brake, electric or hydraulic.
Look for a used one in a scrap yard or on ebay. I spent less than $100 for my system. The controller is the same as used to engage the front wheel drive on the deuce.
Also:
http://www.brakesystemsinc.com/
Closing the valve helps warm the engine on a cold Winter day.