Quote: what makes you grab the tak out instead of say the K or M prime assuming you have one? Whats its attraction?
1) Originally, price. A few years ago M42 lenses, even very good ones were available on eBay for next to nothing. If you did not mind screw mount and a manual lens you could buy a lens whose optics were equivalent to a modern lens costing $1,000, although it would be auto-focus etc. of course. For many, myself included, it allows us to see what a 'pro' level lens looks like since we cannot afford or justify the cost of a $1,000 lens.
2) It is important to remember that there are good taks and not so good ones just like any lens series. Buying an SMC Takumar 55mm f/2 and expecting something special is silly. There is nothing wrong with that lens but it was never intended to be a 'pro' level lens.
3) Appreciation of something made well and made to last. I sincerely doubt that many of the new SDM or HSM lens made today will still be working 30 years from now. And if one of my Takumars has a problem I can take it apart and fix it. I would never think of trying that on a modern lens. A lot of things are like that now. If you are still in high school, perhaps you cannot understand how many things used to be repaired in house. Furnaces, cars, lawn mower, oven whatever, we used to just get parts and fix everything ourselves. Can't do that anymore, everything is electronic and needs an "authorized service representative".
4) You can use Av mode with Takumars, which you cannot do with K or M lenses. It isn't 100% accurate depending on the body and lens but it does work, usually with a little EV compensation. Ira will likely tell me manual mode is better and I won't argue with him, it is more accurate but there are occasions when Av comes in handy and it will work with Taks.
5) Build quality / feel? I suppose that is subjective. I just plain like using the all-metal Taks. Nothing wrong with K or M lenses they mostly have good build quality as well but the Taks just feel better to me.
6) Focus ring. Not a real difference between K & M lenses but trying to use manual focus on a modern lens is frustrating as they were designed for auto-focus with short focus throws. The Takumars have a smooth focus movement that is a pleasure to use.
7) Rendering and Bokeh. Not sure if a generic statement can be made here. Especially as some of the K & M lenses are optically identical to their Takumar predecessors. But if you look at a Takumar photo it has a different 'look' than that taken with a modern lens. Vary hard for me to describe.
But if you already have such good lenses as the K 50mm f/1.2 then I doubt you will see much improvement with the equivalent Takumar. And if you believe that using the M42 adapter is a 'pain' then I would suggest staying with K mount. If you can afford f/1.2 lenses then you have lots of other opportunities without having to mess with the adapter.