If you leave two 50s in a drybox you will find that they breed. Somehow you end up with more than you need without really intending to. In order I acquired
M50/2 (not that great but got it for peanuts just to learn how to use manual lenses)
M50/1.4 from Keh that turned out to have sticky aperture blades
M50/1.4 from Keh that was fine - but they said don't bother to send the other one back
M50/1.7 (because everyone said it is better - but if you already have a 1.4 don't bother)
FA50/1.4 (new, I got a good deal - I sold it to fund something else)
FA50/1.7 (from a forum member, needed some adjustment of the focus ring to make it focus to infinity; OK now, although I think a bit of mechanical wear makes the focus slightly hit-and-miss wide open)
K50/1.2 (excellent, bought it from a forum member, no doubt made money for a thief in Belgium who stole my camera bag on a train out of Brussels)
Do I use them? Not that often - I find the focal length a bit awkward on APS-C, but of course for portraits or isolating your subject they're great.
With the lamented K50/1.2:
Tim