Danny,
There are several different aiming methods in F-Class that I'm aware of, and different reticules can suit each method. For those who hold off for wind etc. on the target rings, then a plain cross-hair or duplex is best I think. If you always aim at a white centre and adjust the knobs every shot, then a dot makes sense. If you want to hold off using the reticule rather than target rings, then a mil dot is okay, but maybe something like a NF NP-R2 with wider spacing between the ticks is less prone to mistakes. Some shooters actually aim at the bottom of the V (or X) ring - not sure what their reticule preference would be. One thing important for those who hold off is to have a long horizontal cross-hair, preferably full width, so that if you do need to aim well wide of the middle, you can still get the elevation sighted correctly. At least one of the NF reticles has no horizontal cross-hair.
Alan