Well, I have a 6. 7 which runs Hot Shot / Courier service, in all weather, currently we are enjoying a few days of warm weather -5 degrees C.
It typically runs a -20 degrees C this time of year. my milage is an average 18 MPG (hand calculated) since the day I bought it June 02, 2007. I now have 70,000 Km or 42,700 Miles. Never missed a day of work thanks to my dealer being proactive with the software and hardware.
No complaints, great truck, very stable in all conditions, sun, rain, snow or ice, Hot or Cold. Always on call, Always starts, Always runs!