the duramax is a good running diesel. The problem tho is on the lb7 duramax, the injectors are garbage. The lly's run hot. Thought I was goin to blow 2 up plowing snow. The lbz's are pretty good. Rods are a little weak, but not an issue till you start reaching big hp numbers. Parts prices are rediculous on duramaxes. The performance shop I work for runs dmaxes, about the same amount of money bullet-proofing my 6.0 as they did on injectors.
Up to 08, I recommend duramaxes for people pulling campers or anything along those lines long distance. The ifs makes for good ride quality and up till then, with the LT package, the interior was top notch. In 08 ford stepped up their game on the interior, so after that its debatable. As far as work, jobsites and fields beat up the ifs parts pretty quick.