The lens works very well for action shots. If you go to my site http://www.lockon.f2s.com
and follow the link for "Taiwan" shots, over 90% of the action shots you see there were taken with the 80-400 VR.
I have had little problem with hunting on the F100. In fact the only time I can recall loosing a shot was when I had the wrong AF sensor selected on the F100... not a fault of the lens. I've put about 40 rolls "through" that lens with no complaints.
I suspect that for very fast moving action like a jet coming straight for you it may not work well, but how much of that do you do?
Optically the lens it brilliant. See Click for large version
Photo © Sunil Gupta
for an example of a shot taken at 400mm on Kodachrome 64.
Use of K25 is best left to static shots because you may loose shots due to DOF problems unless you have perfect light. I have had some success with it on action stuff but it is not worth the aggravation when you get ruined shots.
PS. I have the 300 f4 as well and I now feel it is redundant.