My friend has a used 6.0 and its worked well for him for a long time. He did have to spend some money getting his engine rebuilt (top end maybe? I don't recall now) to get it working good again. Everything else on the truck is fine, so he figured a couple thousand dollar investment was cheaper than a new truck payment. His mileage isn't that great, but he has plenty of power and it starts every time he asks it to start. He is happy with it and has zero plan to replace it anytime soon.
Glad yours is working good for you and hope it exceeds your 300K mark!