I have two F3HP with different focusing screens. One has a P screen, which is exactly the same as the standard K but with diagonal split instead of horizontal. The other has an E screen, whose use can be described by what kitanikon said above - it lets me focus pretty much anywhere inside the frame without getting distracted by the split prism in the middle. P is for when I need critical focus and want it confirmed and E is used in all other situations.
I do still have good eyes (I am 43) so can't really say anything about how all this works with glasses, but there you go.