They can be very good. I have the Color-Skopar 35/2.5, 28/3.5 and 21/4. The 35 I like the best because it is the non-pancake version - which I think is single-coated - and takes great pictures. The 28 & 21 are modern multi-coated. Sharp and contrasty.

The 21mm I have has had some build quality issues: the black paint has worn off in places and some screws came loose a few years ago while I was on vacation (I tightened them and the lens has been okay since.) Also, the frame lines in the external viewfinder started "rotating" - so I took it apart and straightened them.

Over all I'm happy with the lenses. They are much cheaper here in Japan than overseas, for some reason.