I had a Sierra Denali with the 6.0 and AWD. It was a great truck that handled and rode exceptionally well towing and empty. Fuel economy was a different story. 13 city, 15.5 hwy, and 7.5-8.5 towing.
Repairs to the AWD system are not cheap. Wheel bearings are $1800 for the pair and rebuilding the front differential was $1650. All of this was required at about 78,000 miles.
Even with the repair expenses this truck is still my favorite. The only reason I traded it was for more payload and towing ability of a diesel. Otherwise I would still be enjoying that truck.