I love my F6. I have had all of the Fs except the first one. These are my impressions of each:

F2: Best camera I have ever used. I have 2. The only feature I miss on it is auto focus. It has a grand simplicity about it that is unchallenged.
F3: Great meter and very solid. I use mine with the motor drive and the sound is wonderful. It is very heavy though and not good for a walk around.
F4: Very sleek and nice but I did not like the cheap feel and the auto focus is slow. Sold it.
F5: Extremely well made. Extremely everything but unfortunately also extremely heavy and cumbersome. Sold it.
F6: Everything I have always wanted in a camera. Solid F feel, smooth, quiet, fast, lots of customizable options. Pure bliss.