Used on a K10D with an adapter ring, the lens, although quite heavy, balances well with the camera. Easy to focus, with typical Zeiss contrast and color rendering, not prone to flare even when shooting directly at light, and the distortion is very well corrected.
A really fine piece of metal and glass to enjoy.
Now, should really find a cheap T&S adapter for P/K ...
I agree with indianidos, although I use mine with the Nikon D700. Colours are nice, the center is sharp, but if you want to have a lense that is sharp into the corners at - let's say - f 4 or f 5.6 , you probably better look elsewhere.
I used a multicoated 50/4 Flektogon extensively. Among all Zeiss lenses I used to own, the 50 was my favorite lens (well, followed closely by the Sonnar 180).A wonderful sharp lens, absolutely no flare, even when shooting straight into the light. Look for the MC version.