I recently bought both Pentax A 50mm f1.7 ('84-89) for $59 and the M ('77-83) for $29 (both K-mount, this thread title needs to have "Takumar" removed). I have taken quite a few good photos with both. The A is a touch lighter due to plastic parts, but I think maybe the M feels a bit more solid. Optically I don't see a real world difference between them. The A sells for more so is the more desirable lens. I'll eventually sell one and keep the other after doing a side by side comparison, but for the time being I'll just post some pics. Also, if the mods don't change the name of this thread I'm thinking of starting a new one!
The below shots are all taken the A lens (aperture is a guess, but if at all possible i would have kept it around 5 where it's at its sharpest. I would only lower if it if meant not hitting ISO 1600):
Waterfall: ISO 200, tripod, f/4.5
Fallen logs: ISO 800, handheld, f4.5
Bench: ISO 400, handheld, f/2
Log & branch: ISO 200, tripod, f/4
Tree trunk: ISO 1600, handheld, f/2.8
My hands are very steady, I have the PM2's IBIS set to 50mm, but I find it VERY hard to keep this 100mm equivalent focal distance steady and get a lot of blurry shots with these two lenses (as well as with my Oly 40-150mm).