I have a 2007 with Duramax engine and Allison transmission with exhaust brake.
After the first trip towing our 5th wheel with this truck I will never buy another tow vehicle without an exhaust brake. Here's the reason why:
North of Los Angeles on I-5 there is a grade known as the Grapevine. It's 6% to 8% and about 6 miles long when headed north downhill. With my old non-exhaust brake truck I would have to be in first gear and use the brakes no less than 15 times to keep from going too fast and over revving the engine. In second gear I would lose the brakes by the time I hit the bottom of the hill.
With the exhaust brake engaged, the new truck holds our 15,000 lb 5th wheel at 55MPH all the way down the hill. I never have to hit the brakes.
The exhaust brake makes for a very rewarding downhill experience.