I had a 6.4 in an 09 F-350. Good engine.. lots of power, no smoke, very very quiet - almost sounds like a gasoline engine idling. The Power band is amazing - the sequential turbocharger setup gives you instant power right off of idle all the way to about 3000 rpm - honestly feels like you're driving with a 500 cu in gasoline engine by being almost completely lag free. The 5R transmission is practically bulletproof and their strongest to date in these pickups.
Maintenance is going to be it's downfall. You have to pull the cab from the chassis to do the most basic, simply repairs such as injector maintenance/RR, turbo issues, even to do under valve cover wiring harnesses (which will go bad eventually). Open the hood and count the 7 radiators right up front.. yup.. 7.
That being said.... the 6.4 platform is far superior to the disaster that was the 6.0. There have been very few issues (TSB's) as compared to the 6.0L diesel used from late 2003 to late 2007. The 6.0L problems are one of the factors that made Ford and International part company. I still wish Ford/International would have just continued development of the 7.3 (DT-444) which is by far the best Diesel they've ever used.
My lease was up and I decided to turn my truck back in due to the difficulty in maintaining the truck myself - and cab-offs at the dealership (minimum) cost is $500.00 just to get started. If I were you I'd look into the 2011's with the new Ford designed and built 6.7 L engine. Ford designed the cab/chassis so cab-offs aren't required for the majority of repairs you may need to perform down the road.
Should you decide to go ahead with the 6.4L OR a 2011+ 6.7L get ahold of my buddy Matt at
Spartan Diesel Technologies and they can make you a tuner to increase your fuel mileage to over 19mpg. My 09 truck went from 11mpg to just under 21mpg (with even more power) with one of their tuners which leave no footprint on the computer. These guys are ex-Ford and International engineers