I haven't been tracking mine closely, but, it hasn't been bad at all. I have only added 2.5 gallons since I purchased. I have about 5000 miles and am sitting just over 1/4 on the DEF gauge as of now. I am maybe around 750 MPG on the DEF. I guess since I have had the truck for 3 months and 5k, I haven't really paid attention to it since I have only filled once.
Haven't been towing much though. Only 2 RV trips since I got the truck since the wife is expecting and we're sticking close to home for now.
I have read that previous years of the cummins really breaks in between 8-12000 miles and at that point fuel economy goes up and DEF usage goes down. Only time will tell.