I have a III and IV, both with invercones (and ND filter for the III). They work very well and are quick to use. As has been pointed out, the III has rather crowded scales for old eyes! My III is the black-bakelite UK version, Weston speeds and I don't find the film speed easy to change accidentally - it has a small lock. On the IV, there's a greater sensitivity (slightly) and a needle-lock but no film-speed lock.
Word of caution on the models with needle-lock: DON'T leave it locked for extended periods. A friend did that with his V and it stuck the needle to the scale very firmly, the bright paint on the tip is slightly soft and sticky. Not happened on my IV though, although out of habit I leave it unlocked when stored. With all of them, light is a cell-killer so definitely store in the dark.
By the way, the screw adjuster on the back of Westons is to zero the needle, it does not "calibrate" them. Cover the cell and adjust to zero. They either read correctly or they don't - not much to be done if they don't, except get a new cell. My IV over-reads a bit, you can mask part of the cell if they over-read but I just adjust films-speed to compensate.
