If you get one in good mechanical shape with a clean viewfinder/rangefinder, they're great shooters. Very compact, especially with a collapsible lens. Some photographers from that era who used them were Henri Cartier-Bresson, Alfred Eisenstadt, Eugene Smith and Robert Capa (later using Contax and Rollei).
The viewfinder itself is a bit of a peephole, so it's never going to compare to the VF of a later M2 or M3, but it is what it is and I've never found it to be much of a bother. It is very easy to focus as the rangefinder is a 1.5x magnification. Shooting is a bit slower at first with knob wind/rewind, separate VF/RF windows and a different dial for slow shutter speeds, but these things become second nature if you shoot a lot. Some folks never get on with the film loading, but others don't have any problem. It's not as tedious a process as some make it out, especially with some practice.
If the rangefinder hasn't been messed-with, any screw-mount lens from Leica, Canon, Voigtlander or others should work fine. I've heard of folks having problems with some Russian lenses as the manufacturing tolerances may not have been up to par, depending on how much vodka was rationed to the workers that day
. I use an Industar-22 on my IIIf and haven't had any issues, but it's definitely not up to the construction standards of Leica or Canon. They can make nice images, especially if you have one with good glass.
I think Leica lenses are overpriced, especially for older lenses which tend to have problems with haze and/or delicate coatings. Canon lenses are definitely a good value and there are a lot available on the used market. Nikon lenses in screw-mount are highly regarded, but may be more expensive. Modern Cosina Voigtlander lenses are awesome if you're looking for a more modern, higher-contrast look, especially if you want to shoot wider than 35mm.
Stephen Gandy's page has a lot of good information on what to look for at
https://www.cameraquest.com/ltmcam.htm
Also, if you want to buy a good Leica body that has been serviced and you won't have to worry about, try Youxin Ye at
http://www.yyecamera.com/index.html . If you need repairs, his prices and turnaround are very good, generally under $200 for complete overhaul of Leica screw-mount bodies.
When I go to antique air shows, I like to take my IIIf for 35mm, Zeiss Super-Ikonta B or Rolleiflex for medium format and a Crown Graphic for 4x5, if my arms are feeling up to it.
