the SMC Takumar 50/1.4 is considered by many as a 'legendary' lens in its class for sharpness. I believe there was an article on Luminous Landscape awhile back citing it as the writer's favourite lens. I own the lens, as well as the SMC 28/3.5, and the build quality of both lenses is excellent. Silky-smooth focusing, rugged metal bodies with absolutely no plasticky feel to the aperture click-stops. Pentax screwmount glass tends to be excellent, with few exceptions.
Also note that with the M42 mount you also have the option of using Russian glass like the Jupiter 85/2 - another favourite amongst M42 users. You can also attach an adapter to use medium format P6 mount lenses like those available for the Kiev; of note is another 'legendary' lens, the 180mm CZJ Sonnar.
The pentax mailing list, "PDML", is an extremely active mailing list with a treasure trove of knowledgeable enthusiasts. Though discussion doesn't always center around the use of Pentax screwmount equipment, there are several individuals on that list who are very knowledgeable in the area. Another mailing list to check out is "ClubM42" on yahoogroups.