Go and read here: http://www.ford-trucks.com/forums/forum107/ They talk a lot about "bulletproofing" and such. Search on "oil cooler" and "EGR cooler". Basically (I have one so I have personal experience) the 6.0 is a good engine. It was the last generation to use high pressure oil to power the injectors, so it doesn't have issues with high pressure fuel pumps. It has a poorly designed oil cooler which can clog with sediment and fall-out from coolant. When it does it blocks the flow of coolant to the EGR cooler, which then overheats and fails. When that happens, the EGR cooler lets coolant into the intake manifold which causes the head gaskets to fail. If you know all this stuff it becomes childs play to monitor the engine to alert you to know if this is starting to happen and you can do the maintenance required to avoid having a problem.
Other than that, it's like any modern engine, particularly modern diesel engines. It's too bloody complicated and computer controlled to be really considered reliable. A sensor can go in any engine and leave you on the side of the road for the want of a $10 part.
My recommendation is buy the 6.0 if you are prepared to learn about it, maintain it and understand that you might need to fix it at some point, AND you need the towing power, load capacity and fuel economy of a diesel. If you need the power and don't want the fuss, look for a nice V10 gas engine - downside is far worse fuel economy.
Brian