As much as I hate to admit it, the 28-75 might be the sharpest lens I own. It's absurdly sharp. And it's has really nice colors, good contrast and a really nice looking bokeh. It's probably one of the best lenses for the money out there.
Funny enough, I just posted a story pointing to the fact that the build quality is also apparently pretty good, seeing as how mine was on my camera on a tripod this weekend when the the wind kicked up and decided to blow it over. Body and lens both ate sh!t pretty hard. Besides minor scratches, both are fine. See here for story:
https://www.pentaxforums.com/forums/pentax-k-5-forum/178068-i-swear-these-thi...tructible.html