I have a 2005 2500HD 6.0 and we like it just fine. Never had an issue with lack of power and we frequent the Rocky Mountains with the camper. It towed a 7000 pound TT for the first 6 years of it's life and carries a TC now. 112,000 miles so far.
Problems we have had are ignition key cylinder has failed 4 times. I carry a spare now. Blower motor resistor replaced. I did the pump rub fix even though it wasn't leaking just for peace of mind. I found a little wear in the transfer case and the clip that sometimes falls off was still where it's supposed to be. Both front wheel bearings replaced @ around 100,000 miles. Replaced the #2 knock sensor. Three broken exhaust manifold bolts. I can't think of any other issues off the top of my head.
This truck has not had issues with rusted brake lines (and I live in the rust belt) or the instrument cluster going wonky. I recently had the front end aligned for the first time and the shop found no issues with the front end parts.
All in all it's been a good truck. The thing I like best about the truck is how easy it is to work on the engine. No problem getting to everything including all of the exhaust manifold bolts (although I hear the CPS is a real pain to get to).
Highway mileage is 10 to 13 carrying the camper and 14 to 17 empty.
Hope this helps.
Mike