The mentioned K 3.5/28, 2/28, 2.8/30, all quite special. The 2/28 manages to have the same optics as the Distagon in a lighter and smaller, nevertheless very strongly built body. If only sharpness the K 2.8/30 doesn't really outperform my Pentax A 2.8/28 but 'has something', the A lens is also very good too though!
- one could rather ask, which Pentax K is 'not' interesting?
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there are 3 lines of manual Pentax lenses with K bajonett:
first is the 'Pentax K' line, also referred to as 'Pentax SMC', next the 'Pentax M' and last the 'Pentax A' lenses. The 'Pentax K' have very strong build, one feels the Takumar legacy, many manual focus enthusiasts prefer them over the later 'M' and 'A' lenses. The 'Pentax M' are mostly smaller and lighter, the 'Pentax A' have electrical contacts so that one can choose aperture on the camera, their aperture ring is a far cry from the smoothness of the earlier lenses but clunky. Actually there is a forth line, the K mount Takumars which should not be confused with the M42 Takumars, they are uncoated budget lenses and there are just a few them.
many gems of even earlier Pentax lenses, the Takumars in M42 screw mount, specially some very early lenses from late 50s and beginning 60s as e.g. the rare 58mm lenses mentioned by mfogiel, but also later S-M-Cs like the 1.4/50, 1.8/85..
Any 55mm lens, if Takumar or Pentax K, is very good and costs very little
The Pentax Lens reviews on Pentaxforums are a very good source for information:
http://www.pentaxforums.com/lensreviews/