I own several M42 lenses, and I'm happy with them all!
S.M.C.-Takumar 28mm f3.5, a very sharp lens.
Super-Takumar 35mm f3.5, also very sharp, small in size and lightweight.
Super-Takumar 35mm f2.0 (old version), very big and heavy, but quite good, apart from quite visible vignetting at the corners.
Super-Takumar 35mm f2.0 (new version), much smaller than the older one and better too.
Carl Zeiss Jena 35mm f2.5, very sharp and high contrast, but mechanically poor (sticky aperture, a common problem with CZJ lenses).
Super-Takumar 55mm f1.8, sharp, but a shade is essential. Prone to flare.
S.M.C.-Takumar 55mm f2.0, sharp.
Super-Takumar 50mm f1.4, a very nice lens when used with a shade. My favorite 50mm.
Fujinon-T 100mm f2.8, very compact and sharp.
S.M.C.-Takumar 135 f2.5, I haven't used this lens much, but it looks good!
Some M42 lenses are very pricy. F. ex. Pentax lenses wider than 28mm and the 85mm f1.9/f1.8. In general most Pentax M42 lenses are getting more expensive, but there are still bargains out there. Especially non-Pentax ones.