I can't speak for the duramax. I have the 6.0l in my 2500HD. I love the everything about the truck except the gas mileage. With the duramax, you won't have that problem.
My company owns the GMC dealership in town and I have become friends with several of the mechanics. The mechanics were not happy with the generiation 1 Duramax because the location of the injectors were very poor and a pain to replace. Generation 2 of the engine fixed the issue of the injector location and improved the injector themselves. They no longer complain about the Duramax.
Are you looking at new or used? If new, the '06 models have a new 6-speed transmission that is supposed to be SWEET!. It allows you to go into manual shift mode when towing to get the performace you are looking for. Supposed to be really nice for people pulling heavy trailers.
I have a 2002 Duramax Crew cab with a 5 hole chassis mount on it. The box weighs ~ 2000lbs loaded and I get between 15-16 mpg. Never had a wrench on the engine other than normal maintenence. I had a extra leaf spring put in for the weight of the box and to pull the travel trailer. I also put a 45 gal after market fuel tank on it (Transfer-Flow) so I have about 600 mile range for travel. Was handy in Alaska where it could be hard to find cheaper diesel. It runs at 80mph without any drop off in mileage and I am really pleased. With Allison transmission be sure and change the transmission filter at least every other oil change (word from a guy that builds the transmissions). Wouldn't trade it for the Ford or Dodge. Good Luck.