I love my IIIf. I don't shoot 35mm film much anymore, but when I do it's usually with the IIIf.
1. Loading film is a complete pain, and takes a good long time.
2. The viewfinder is small and dim, and does not compensate for parallax. The focus window is separate from the framing viewfinder, and many IIIfs suffer from degraded silvering on the rangefinder mirror, making focus even a bit more difficult.
3. The available shutter speeds are different than the ones in standard use today, so you have to think for a few seconds every time you set one.
4. The shutter speed dial spins when the shutter releases, so you have to be careful where you put your fingers.
5. The film winding controls are all small and awkward.
6. Any lens other than 50mm will require an accessory viewfinder.
7. No built-in exposure meter, of course.
But, In my opinion, the IIIf is a magnificent camera. I paid $180 for mine in 2003, with a Summicron. I'd gladly pay twice that now for the same camera.
1. If you have one of the better lenses, the images are superb. Certainly not as crisp as you might expect from modern equipment, but still very pleasant, and they look very different from those taken with more modern equipment. If you must have a modern lens, Voigtlander screw mount lenses are easy to find on eBay, and are excellent.
2. The camera is quite small, even compared to an M, and especially easy to fit into small places if you have one of the collapsible lenses (I prefer the Summicron f/2).
3. It's such a pain to use one of those things that you aren't likely to go snapping away carelessly, and since you have to think about what you're doing every time (Film advanced and shutter wound? Aperture set correctly? Focussed?), you tend to end up with a lot of keepers. At least I do.
4. If you get a good clean-lube-and-adjust (CLA) done on it by a competent repair man, it will probably still work 50 years from now. All current digital cameras will be completely inoperable well before it will need its next CLA.
5. Also makes a great conversation starter when you're around people older than 40, especially photographers, or veterans who were stationed in Europe in the 1950s (admittedly I'm speaking from an American perspective there).
6. There are few other tools you can buy that have such a fascinating history.