See: http://www.optyczne.pl//index.php?test=obiektywu&test_ob=234
(Google-translated English test report here)
The summary and sample pictures page is at the 11th page.
Well, the test results mostly coincidence my practical (limited) experience with the lens, i.e., the (barrel) distortion is obvious and the image corners are not sharp. The small size and the high quality build of the lens have made it a joy to use, though.
Nonetheless, the test mentions about the accurate focus of the lens on K-5, which I found the opposite with my K-x when I used the DA21 lens, when mis-focusing often occurred, in particular when the lens was used wide opened and for close objects.
Finally, for what it is not mentioned, I particularly like the colour rendition of this lens, which represents much the old Pentax taste and natural style, which might be the true reason for why this lens (the resolving power is not, for which it lacks of, at least not across the whole frame).
Read Also:-
RiceHigh's Subjective Ratings on Pentax AF Lenses on Pentax DSLRs 2011