I have just over 5,000 miles on mine. Averaging about 16.8 based hand calc'd. I do a suburbs to downtown and back drive every day, so, a lot of stop and go. Pure highway driving is around 21. Regens really kick the MPGs way down. I'm averaging a regen every other tank or so (maybe every 900 miles-ish). Haven't towed anything since I have been tracking with fuelly.
I was averaging 15.6 in my F150 5.0, so, doing a little less than 10% better which doesn't quite cover the fuel price difference. BUT, that truck didn't work for me towing, so, kind of a moot point. In any event, I was hoping fuel cost would be close to a wash in the switch and I think it will be in the long run.
The gasser will be cheaper to buy and own, I don't think there is a lot of debating that. However, I don't agree with the math that was presented above... First... most people don't pay MSRP, you'll likely get 10% or more off of that $9100 upgrade, so, call it an $8200 option. When you sell or trade your truck, you recoup some or all of the initial upgrade cost. Unless you are purchasing the truck with the intent to never sell/trade it, the equity has to be considered as well. The higher mileage, the more gap you will see... If you are going to keep it 2 years and trade with 40,000 miles, you probably won't recover the entire $8200 upgrade... but, if you drive it 6 years and 120k, you're going to widen the gap from the same gas truck. 10 years and 200k, and the truck is probably worth more than $8200 more than the same gas truck. Certainly you will have spent more maintaining it, but, you don't need to drive it millions of miles for it to make sense. Not a whole lot of people looking to spend good money on a gasser with 200,000 miles while the cummins is still chugging along for many more miles and has a ton of value left.
Beyond that, there is a "want" factor than no one can figure out for you. I went with the cummins because I wanted it. A gasser would have worked for me, but, I am paying for it and I am the one that gets to make that decision.