This lens is a blast!
I've had one of these lenses for a number of years, using it first on the K10D, then the K-5, and now the K-1.
On the K-1, I'm happy with the overall result although the CA in the shifted corners is worse than the K5 (as expected).
A polarizing filter (the 28S has 77mm threads in it's built-in hood) does vignette on the K1 at >10mm landscape shift and >7 mm portrait shift. There's no problem with physical clearance between the lens and K1 body so one can shift in any direction.
The trick with OVF metering the 28S is that it has to be unshifted to meter properly due to limitations in the design of OVF focus screens. OVF issues also imply that composing on a heavily-shifted setting often requires the aperture to be fully open.
My workflow with the 28S is usually:
1. Unshift, open the aperture, compose the frame, close the aperture, green button meter (I may take a test shot and chimp the histogram).
2. Open the aperture, shift, recompose, close the aperture, shoot.
Although it feels like a lot of steps, it goes very quickly once you've done it a few times. The two-ring design of the aperture controls makes it easy to set the desired shooting aperture with one ring and then quick-slide the working aperture between full-open and chosen-setting for the various metering and recomposition steps.
P.S. I'm surprised more portrait and street photographers don't use shift lenses for full-length portraits. Using a regular lens and angling the camera downward to get head-to-toe filling the frame results in a weird keystoning effect that makes the subject's head and shoulders too large and their legs too small. With a shift lens, one can hold the camera level (and at eye level) and then shift the frame to get head-to-toe.
A couple of threads that might interest you are:
A Bit Of A Shift On The K1 - PentaxForums.com How many tilt shift lenses are available? - PentaxForums.com