Mine bone stock saw as good as 19 or so MPG on the freeway. I have since deleted the DPF exhaust and added EFi Live. I see 23-25mpg on the freeway. My current tunes are Eco, Light Tow, 75, 100, and 150. I haven't had any issues with it thus far. I just hit 90k miles a couple days ago and bought it with 85k on the clock. The previous owner is a member on the boards father. And the only issues he had was the Lift pump when bad and the fuel tanks straps broke and lost the fuel cell. All was covered under the warranty. Like people have said the tranny is fantastic but anything over the 150hp tune they aren't loving life. My truck pretty much always stays on the 75hp tune and its a blast to drive. Only thing I don't care for his the small tires and wheels it comes with from the factory. Nothing that some leveling and 20's can't fix haha.
Also on DuramaxForum.com the guys over there say to delete the EGR, re-route the PCV, and upgrade the liftpump and it will eliminate damn near any issues. I have the EGR closed in my tune but have also bought a plate ($12) to install. The PCV is like $20 in parts, and the liftpump is generally $600-650.
The DPF exhaust has to go in order to see some good mpg as well. Do some searches on it. I got my full exhaust for $150 and paid $650 for my EFi Live AutoCal preloaded with the tunes from the go-to guy for LMM's.