I had both the F4 and (for a brief period) the F5. I sold them both and went back to the F3, and more recently, the F2. My reasons:
1) I don't need autofocus (my only autofocus glass are my DX lenses for my D80)
2) You can strip them down to a small body (take off the motor)
3) Smoothest manual film advance EVER made (F3)
4) Not battery dependant (F2)
5) Meter with any lens Nikon makes or has ever made (except G series lenses)
An added advantage to the F2 (especially with an MD-2 attached) is that you can use it as a blunt weapon in a bar fight, then take pictures of the aftermath!
I haven't bothered with an F6 (and won't) because of the lack of interchangable finders. I actually USE my waistlevel finders quite a bit, and I love the DA-2 Action Finder for the F3.