(1) There are two versions of the M 28/2.8, so in discussing this lens, some care needs to be taken as to what version is under discussion.
I have, from memory, the later version (I'm not at home as I write this post). Colours are good, sharpness is ok but not stunning either. Mine is mis-calibrated on the focus scale so true infinity focus is around the 10 feet marking. So if you rely on the scale for a distance shot without being aware of this, you are focusssing beyond infinity and sharpess and contrast suffers. It took me a little while to realise this. From conversations on this forum, I am aware of another member whose M 28/2.8 also appears to be mis-calibrated. In a quiet moment, I'll look to re-calibrate it. It is a risk that these older, simple construction lenses have been pulled apart at some stage of their lives.
(2) I also have the M 28/3.5 which I have found to be a lovely lens (good colours, sharp) and exbibits almost no bad behavior (ie no PF or CA, good flare resistance). I used it for Single in September 2012 and found it a joy to use.
See this link for my album:
southlander's Album: Single in September - PentaxForums.com
The album consists of my daily best shot with the M 28/3.5 for 30 days. In aggregate, I took approx 900 photos with the M 28/3.5 over 30 days, so I think its fair to say I got to know it well. I tried to use it in a wide variety of settings over that time.
Bottom line: The M 28's are plentiful and not expensive, but I have a strong preference for the 3.5. The 3.5 particularly shine with blue tones, IMO.