While I don't personally own one, I have two close friends who have them. They both agree the 18-19 mpg is about right, BUT, they both carry modest loads, do a lot of city/highway combo driving and neither "baby" their trucks.
The big thing for both of them is the towing. One friend is in construction and frequently towes a heavy trailer with a large Skid-Steer, the other towes a 5500-6000 lb camper. Both say their trucks tow better and much more economically than their former 5.4 litres. A lot better gas mileage.
I am just putting the finishing touches on my share of my second consecutive kid's college education. When we're finished, I'll be shopping for one myself.
Hope this helps...
Jon