I have a Leica IIIa and IIIf as well as an assortment of Zorki's and Kiev 4's. I have two Zorki 1's which were in good condition to start with and after adjusting the shutter I get good results from them. Compared to the Leicas, however, they really have only three usable shutter speeds, 1/100, 1/200, and 1/500. I get too much shake to use take hand-held shots at anything slower than 100. I can shoot hand-held with my Leicas at speeds as slow as 1/25. The Zorki's have no shutter brake to keep the curtain speeds uniform across the frame so, unless you adjust the shutter very carefully, there is a tendency to have more exposure on one side of the frame to the other.
My Kievs are closer in performance to my Leicas, but I tend to use them less because they are larger and heavier. A Leica III or Zorki 1 with a collapsible lens (5cm Elmar or Industar 22) will fit in my pocket, making the camera ideal for walking around in a city. Mechanically, my Leicas are much smoother and feel more refined than my Zorki's. If you compare a properly CLAed Leica IIIx to a Zorki, you WILL notice the difference.
As with all FSU cameras condition is always a concern. I have been lucky with most of the ones that I have bought but one of my four Kievs has a light leak which I have been unable to track down even after taking the camera apart three or four times. Shutter ribbons on Kievs are another failure point although I have not had problems with that.
If you are simply looking for a pocketable rangefinder which takes interchangeable lenses, a Zorki will probably work for you. If you have your heart set on a Leica, save up for it. Checking the prices at KEH, Leica IIIx's are not that expensive.