I agree, my girlfriend has a few of them in her corporate fleet and they can't kill them. They have a 99 F-250 7.3L with 750k miles and it is still going strong on the original motor (seems like it needs a transmission rebuild about every 250-300k miles though).
The 6.0's didn't last that long and the 6.4s seem to be a bit better than the 6.0, but not by much.