I have been using the Pentax K 28mm f3.5 as my standard lens on the NEX 6 recently and have found it to be superior to the alternatives that I have tried.
First the drawbacks. It is heavier and larger than both the Pentax M and Zuiko f2.8 lenses, while it uses a 52mm filter thread, rather than the standard 49mm of most of my heritage glass. As you might expect, the f3.5 aperture is not quite as bright as the f2.8, but it's quite usable.
I've not done exact comparison shots, but image sharpness/contrast is not a problem, probably similar to the M f2.8 which is pretty good. If we ever see the sun again, I might make a few comparisons.
One of the main advantages of the K lies with its control of CA, it's hardly noticeable on most shots. I don't think that I have had to correct a single image for CA, while this was very much the Achilles heel of both the M and Zuiko lenses.
Another big advantage over the Pentax M is the lack of barrel distortion. I've not measured this scientifically, but I've just not noticed it on shots where it could be a problem. I did photograph a brick wall as a test and I couldn't detect any, so, in practical terms, distortion is not an issue.
Conclusion, the Pentax K f3.5 28mm is the best of the bunch to date!
Update - just taken a look at Ebay and the last Pentax 28mm f3.5 K to sell went for £102 plus postage! I need to look after what has become an appreciating asset......
Further update - As I take more photos using this lens I get to know it better. It's not completely free from chromatic aberration, but it needs a pretty extreme situation for it to be a problem. Much, much better that the f2.8 Pentax M and Zuiko in that respect.