I got three FSU's a couple of months ago, a Fed 3(a), Fed 3(b), and a Z-6, all in nice shape with the Fed 3(a) having the most (many) miles on it. After using all three for this short time a couple of things jump out: the 3(a) was the quietest and smoothest, maybe because it was the most heavily used of the three by far. However, when Yuri at Fedka got done with the 3(b) IT is now the quietest and smoothest, and is a nearly new camera. The Zorki 6 has the brightest viewfinder (the Feds are ok, just not surprisingly bright like the Zorki), and the Z-6 also has a rough, sticky shutter button. The shutter button position on the Z-6 is also not nearly as comfortable as on the Feds, either. Located on the center post of the winder lever, it doesn't "fall to hand" naturally as do the releases on the Feds. My Z-6 is practically new, with no indication the body was ever actually used, except the case has some scuffs -- although that might not be the original case, either. I'm sending it next week to Yuri for a CLA, and fully expect he will have the shutter release working butter-smooth when I get it back. With it's excellent viewfinder, I think I can get used to the awkward (to me at least) shutter release position.
I think these FSU cameras suffer more from shelf disease and lack of use than from anything else. The older and more heavily used they are, the slicker and smoother they seem to work, which maybe makes perfect sense for a piece of precision machinery. Next time I think I'll ignore cosmetics and worry most about finding a good, well-brassed and heavily used camera!