Two of my sons have them. The one is a 1999 and it gets 8 mpg empty. Load it and it gets 4-5 MPG. It runs fine and we have had it to several GM dealers/mechanics and all of them say there is nothing wrong with it. It only has 38 K on it for miles.
The other one is a 2005 and he gets 12-13 MPG empty and 8-10 MPG with a trailer.
Both truck drive nice and pull well. The fuel mileage just is terrible on them both. You can't afford to drive them very much.
I have four Dodge Cummins trucks. The 1990 is a five speed and it will get 18-19 MPG empty and never under 13 loaded (It has 397,000 miles on it too). The 1998 and 1999 are a little worse in MPG but never under 12 loaded and usually 16-17 loaded.
I just picked up a 2004 extended cab out of Texas. It only has 12,000 miles on it. I have not drove it much but it is not getting as good of mileage as the others. It does have a little more power.
All of them are manual transmissions. So if the newer one will last as well as the 1990 I will not need to ever buy another pickup. I do keep my eyes open for any low mileage first Gen Cummins that come up for sale. They are just about bullet proof.
My brother has the Chev with the Duromax in it. It is a great running truck the fuel mileage on it is not real good either, 13-14 empty and 8-10 loaded. So I would not spend the extra money on one of them over the 6.0 gas. The mileage is not enough better to justify them. Plus the 6.0s are selling cheap if you can afford the gas in them. My son's will last a lot of years because they hardly ever get driven. They come and swipe one of my Cummins to pull anything heavy. LOL.