As one who owns an M3 and a Kiev 4a, I must say that when my 1969 Kiev arrived from Ukraine in 2001, and for a whopping sum of USD $35, I was blown away at what it could do. The pictures are very good coming from its Sonnar Copy 50mm f2 Jupiter 8M.
The M3 does indeed win on handling/ergonomics, film spacing, precision of fit and finish, smoothness of mechanical operation, quietness of shutter, lack of body motion from shutter, and corresponding pride of ownership.
That long base rangefinder on the Contax/Kiev is excellent, but the viewfinder is miserable otherwise. The M3's finder is so much better than that wee little peephole excuse for a finder on the Contax/Kiev. I mitigated that finder problem with a Voigtlander Kontur, which allows one to shoot with both eyes open.
And that "Contax hold" is a bear. All that said, I doubt if most folks could tell the difference between most images made with my "Kiev 4a + Jupiter 8m" and my "Leica M3 + Summicron."
About a year into my use of the K4a, a ribbon came loose from my winding it too fast. I got some new silk ribbon from Oleg Khalyavin in Moscow. With that and instructions from Rick Oleson's website I replaced the ribbon. It has worked ever since, with no surprises. I have read that is about the only failure to expect from a good Kiev 4a. I read that it may need ribbons every ten or fifteen years or so in normal use.
A Kiev 4a ain't no Leica M, but honestly the pictures are stunning, IMO. And it really is a fun experience to make the thing deliver them. Despite its 1936 Zeiss Contax heritage, and Soviet era reduction in fit and finish, it nevertheless gets the job done well and in good style.