I've got a 2011 F250 with the 6.7 diesel. I had to add DEF at around 5k miles and am currently at just over 8k miles. It wasn't that big of a deal, and the warning comes on long before it forces you into reduced speed mode. I don't carry DEF with me regularly, but if I was planning a remote trip, I definitely would. Fuel mileage is all over the board depending on what kind of driving I'm doing. Stop and go city driving ranges from under 10 MPG if the engine's cold to maybe 14 max once it's at operating temp. Driving on the freeway at 60-65, I can usually get around 22 MPG, sometimes a little more. Freeway at 70-75 drops me down to about 18 MPG. I got 12.5 MPG towing an 8000 pound boat up a mountain pass and back down the other side. Considering that's still better mileage than I get on a short trip in town from a cold start, you can see why it's not a good choice for a daily driver, thus my 8000 miles over the 15 months I've owned it.
One thing Ford has had issues with on the 2011 Super Duty (not sure about 2012's) is the transmission. I had to take mine in twice to resolve "shift flares" where it would essentially slip out of gear from 2nd to 3rd until you let off the accelerator, usually after a cold start. They finally seem to have identified the problem though. The last time I brought it in, they replaced solenoids in the tranny and I haven't had any of the shifting problems since.
Scheduled maintenance is very limited for the first 150k miles. Basically just routine stuff like oil changes, fuel filters, air filter, tire rotations, and fluid checks. At 100k miles, it calls for changing the rear axle fluid, then changing the coolant at 105k. After that, nothing special is required until 150k when you have to change tranny, front diff and transfer case fluid, as well as replace the accessory drive belt. Some of those get bumped down to 60k miles for frequent towing/camper use, or 50k for off-road operation.