I also have a Duramax. I have a 2006 (LBZ). I bought mine with 258k miles, and now I have 267k. The only things I have replaced is the alternator and the water pump. Talk about being a bitch to work on! Took me 8 hours to do the water pump, but was well worth it. I have checked my injector rates and they are all perfect.
I like my truck. I like Cummins too, but I am a GM guy and I prefer a GM truck over a Dodge, so I am stuck with a Duramax. However, I love the Duramax and don't question it's capabilities one bit.