Just shot my first roll with the M20/4. I've taken a few shots on the K10d as well. I posted those shots in my review
Pentax Lens Review and Specification Database - SMC Pentax-M 20mm F4.
A couple of posters asked for digital. I will do some more shooting on digital, but I have to honestly say that of the 6 ways I have to get to that focal length on APS-c, this would be in the lower half in terms of my preference. At a maximum aperture of F:4, the focus on this lens indoors takes some getting used to, and then there is the "green button" exposure on a DSLR. Perhaps I have been spoiled by the DA21.
At current prices, I would not buy this lens over the DA21 if I were using it primarily for digital. A nice copy of this lens usually goes for $400 now on Ebay, and a like new is $500. I bought mine for $300 (the cheapest I've seen), and it is a well worn but well-functioning example. The DA21 goes for $500 brand new, and less if used. For that difference, if any, in price, I'll take a sharp, autofocus, autoexposure lens with a half stop more speed.
However, as an ultrawide on film this lens is a blast. From F:8 on, it is plenty sharp corner to corner, and the color is superb. In addition to the review, here are some more examples from my visit to the farmers market. (Fujicolor Pro 160s)