I have an 07 Classic and wouldn't trade for it. I like it much better than the newer body styles. I have 45k on her and it's a pulling machine from a heavy 20' gooseneck stock trailer loaded down to my 20' Skeeter. Running 70mph I am getting 17mpg pulling the "Bug".
Just cleaned the K&N air filter so I may get a bit better,,,it was nasty. I also have a Super Chip reprogrammer. It makes a big difference.
Don't bother with the air inductions or air cleaners. If you ever go to the dieselplace.com they have proved many times over that air filters give you no gains.
If you can get that mileage out of your duramax you are not in the norm. I had several mods to improve mileage on my Duramax and tried the top 3 programmers, and no matter what you will not see an increase in mileage with them when hand calculated. The programmers do however throw off the DIC mileage if you are going by that.