Have an '08 F250 6.4L diesel,4x4 w/ 3.73 gears. Usually get 14 MPG mixed driving, 17+ cruising on the highway empty. All stock, nothing added. Super quiet, plenty of power, fun to drive.
(Many) others have claimed low 20's MPG with a DPF delete and tuner installed, but no first hand knowledge.
Early ones had some issues with radiators leaking (warranty covered), leaky injectors (current recall on affected units, covered under warranty). As I understand, nowhere NEAR the problems the 6.0L had in it's first year out.
If you look hard enough at any brand/engine, you WILL find problems. Yes, EVEN with Toyotas and Nissans.
Zero problems with mine, had it over a year now. I love it and would definitely buy another one. The DPF cleaning (or regen) is a little weird at first, but easy to get used to, once you understand what it's trying to do.
15 MPG average with an 8000# diesel truck is better than 11-12MPG with a v-8 gasser 1/2 ton for me. Pulls like a tank,too!
Good luck with your purchase, whatever it may be.
Tim