I've owned a 2000 7.3L, a 2004 6.0L, a 2008 6.4L and about to own a 2011 6.7L.
I can't say much about the 6.7L yet, but by far the best motor I had was the 7.3L. That truck took a beating and just kept going.
The 6.0L wasn't a terrible engine, but like any new build, it had its issues. Mainly the turbo. Once I hit the 70000km mark, it seemed like there was a valve or a hose or something that would need to be fixed every 4 to 6 months.
The 6.4L has been good. The only thing that I noticed that there is a significant decrease in power when the air filter is not clean. Never had that drastic of a power loss in any other diesel.