Takumar 200/3.5 this month. I bought this to upgrade my M200 with something sharper and better. Optically this does just that. Size and mount are where I have complains and the reason why this doesn't get so much use. My M*300 is about same length and K mount, which make it easier to use and it has more reach as a benefit. M150 and M120 give little less reach in much smaller paggage. I find the use of M42 adapters a little problematic. Flanged adapter is easy to use, but you lose the focus to infinity. Original adapter is pretty easy to put on, but harder to take off, so I don't like to use it, if I don't plan to shoot all day with M42 lenses. Also my cheap adapter is a bit loose in the mount, which make shutter shock worse. Original Pentax adapter might solve this.
Still this lens is really impressive and I'm going to hang on to it. It is very sharp. Even wide open. It renders differently from modern lenses. I really like the way it produce fat wide lines in OOF highlight edges. I found them really cool. Also 18 blades provide round bokehballs even at min aperture f22. It also produce nice spikey starbursts. There are also classic fringings in high contrast and heavy loss of contrast when shooting against light. All in all not an easy lens to live with but in right conditions great results.
The new thing I learned this month was that this lens really is worth shooting at f22. It doesn't loose much of it's sharpness and you get best starbursts that way. As I tried to live with flanged adapter the f22 also gave me reasonably sharp results much further away than my focus could reach.
Also I found out that there are two version. Mine is preset and other is auto aperture I believe.