The 'soviet' 35mm finder I have is very bright and clear. Mine has some masks that can clip on the front for 85 and 135mm views, too.
However, the angle of the 'foot' that sits in the shoe seems wrong. The finder tilts down a little too much [like a parallax correcting finder set for 1 - 2m], I think. I haven't tried it on my Leica, but shots taken on my Zorki 2 have slightly random framing as a result. It's possible the shoe on the Zorki is wrong, but looking at the finder I think it's the base of the finder itself.
Shame, as the optics are pretty good. I'll probably get rid of the finder as, for framing, the turret finder is more accurate.