We had heard to stay away from the '03-07 6.0 engines. So when we went looking in 2010, we picked up a used '08 F350 diesel. It had 47k miles on it then... we had a few "water in fuel" issues the first year, but finally got the dealer to replace the fuel filter and some other fuel line parts which they said were pretty well gummed up (we think it was used as a farm truck before we got it... it has a definite "working-man" aroma to it at times). Since then, it's been great (knock on wood). I take it out at least once a week for a 30mi highway trip (15 miles one way, with a 2 hour band rehearsal in the middle) to keep it lubricated. Including those miles, it gets about 5k miles on it per year.
It gets better mileage than our '95 gasser F250 did... that one drank at a rate of 9.5-10.5 mpg no matter how we loaded it (truck slide-in camper included). When towing the 10k Elkridge, that dropped to under 7mpg. The F350 runs closer to 10mpg towing and 14mpg average. I have taken it on some all-freeway trips (not towing, lightly loaded), and got up to 18mpg, which was nice. Not much under my daily driver Subaru (2000 Outback, 23mpg average).