Depending on the configuration, the lower end of the GVWR on the F-650 is 25,600lbs, while a CDL is required for anything above 26,000lbs. I’ve definitely seen a couple of people with them as “personal” vehicles, but they’ve all been show cars of some sort.
I usually see the F-450/550/650 tow trucks and the sort. I’ve also seen some fire departments with them for some specialty type vehicles.