I have the '05 6L Sierra Denali (see profile) and am getting about 16.5mpg in mixed driving, 18-20 HWY and about 10-11 towing. My friend had the '04 with Quadrasteer and got about 13 with a heavy right foot. Keep in mind these are AWD so the milage is poorer than a 4WD with 2WD option.
As mentioned earlier these trucks turn better than most cars. The tow mode on the 4 wheel steering allows the truck to crab when changing lanes, very cool. We towed loads of Christmas trees with his SD at or above the 7,000-8,000lb limits and it worked flawlessly. I wish they would have kept it when they switched to the Crew Cab.