A sparkling diamond from Carl Zeiss Jena - this is not only a beautiful looking lens, it also performs optically very well. This is the digital age, but if you give it a try on medium format slides you'll be rewarded with eye-opening results. It is compatible with Pentacon Six, Praktisix, Kiev 60, Mamiya 645, Pentax 645 and Contax 645 cameras using a single adapter ring easily available on Ebay. I did try to use it on a 35mm DSLR, but it's somewhat of a silly idea, as a lens as big and heavy as this doesn't give any extra performance. Some people use it on 35mm cameras with a tilt shift adapter, something I haven't tried yet although it sounds interesting. The lens is uncommon but not particularly rare. Mechanically it is very well built; excellent construction and top of the line optical performance. I have all the other versions of this lens, they are also excellent performers.
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 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.
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.