I purchased the 2012 2500 6.7 Laramie in late Feb. I am currently getting "real" MPG at 14.0+, 4200 miles. I would suspect it will increase just as most Cummins engines do and land around 16/20 cty/hwy. I had a 2003 Ford 7.3 and it was at 15+ avg after 120k miles, so this truck should do better and the bonus of having more power which is what I needed. MPG is not as large of a concern as towing ability, but fuel prices do open these questions. For the price and towing abiltiy it beats Ford and GMC at about an $8000 price difference so that could be considered a few tanks of fuel and the difference in fuel MPG between the others is not enough to sway me.
Good Luck and hope that helps.