K 35/2
This lens has been my favorite prime lens for a long time on film camera. Especially on events. On a digital crop camera it becomes a std. lens (±50mm). Although it's very usable on the digital camera's, thanks to the green button for stop-down metering, I haven't used it a lot since I've got the 16-45mm zoom lens.

Permformance.
As you can see below the center performance is great even wide open, but the corners are an other story. Although they look bad, in real life it's not that bad on a crop camera. Especially because I looked at the extreme corners for testing. The only bad thing is that the corners are not getting a lot better when stopping down till f4. Overall this lens looks a bit like FA50/1.4 in performance with better center performance. After testing I was a bit surprised about the good center performance, I forgot a bit how good this lens is.
On a FF camera the corner-sharpness is of course worse. And then it needs f5.6 to get sharp corners, but as mentioned  before I didn't find it a problem in every day use.
Distortion is very good, so usable for landmark photo's.
CA is among the best getting a 9 and contrast is just a bit lower then on the FA50.

This is a very rare lens and I need to note that the M35/2 and A35/2 have a different design (7-7 vs 8-7 elemnts/group) and they are much lighter.
If you find a good sample and don't need AF, this can be a good lens. The FA35/1.8 and DA35/2.8 macro are probably a lot better and expensive. There is also a FA35/2.0 AL which must have better corner performance thanks to the Aspheric-lens element.