I have M 28mm f2.8 (first version), which is supposedly one of the worst Pentax-brand 28mm lenses. But I find it to be acceptably sharp and fun to use. So I doubt any of the other 28mm Pentax primes would be terribly bad. To explain, my M 28mm is not as sharp as the DA 35mm f2.4, but it is sharper than the 18-55mm. With digital PP, it is sharp enough.
I was thinking of getting a 24mm, but there are no current AF 24mm primes on the market for Pentax (or 28mm!). The legacy ones are all fairly expensive (except for the Sigma, which you might find?). With the help of a nice Pentaxian I got a DA 21mm a a reasonable price, and am enjoying it so far. So, for APSC camera, that is another decent choice. It is not brutally sharp, but it is decently sharp, and it has a very nice style of rendering. The DA 21mm is a bit sharper than the M 28mm, maybe not quite as sharp as the DA 35mm, but it has a
much nicer rendering. The DA 21mm has that Pentax limited magic
According to all the research I did, in an ideal world, I would get an F 28mm and DA 21mm, then maybe a manual 24mm (The FA 24mm seems to have some flaws on digital, and it is expensive). Samyang makes an interesting 24mm f1.4, but it is not as affordable as some other Samyang primes. It would complement the lenses I mentioned well, though. Anyway, my budget is depleted, so I will stick to my old manual 28mm and have fun with the new 21mm. Together with the Samyang 14mm I have, these cover the wide angle range very well.
Honestly, many Pentaxians are waiting for Pentax or at least some other lens maker to produce AF primes in 24mm and 28mm focal length.
Edit: Oh, one more thing. There is a thread titled "Fun with any 28mm lens" (
here). It contains nothing but photos with 28mm primes. You might want to check it out to help decide what to get