I have a 2008 F-350 with a stock 6.4. Been across the country 3 times with a heavy camper towing either a boat or a Jeep, other than a water pump failure it's been flawless.
I've had a couple of radiator leaks with my 2010 but otherwise it's been a good motor. It just struggles pulling my ~15,000lb Momentum up the hills here in Colorado, and it's thirsty (I'm happy if I average 9mpg on a round trip).