The Bullseye was the prettiest camera I ever had, but the most difficult and annoying to use I ever had. Great lenses, but horrible cameras to use. Everything about them said luxury and well built though. On the other hand, the Leica M2 and M3 also had that feeling of extreme engineering but their user interface was a masterpiece and the lenses were nearly as good. Which is why I didn't keep the Bullseye for long and why I still have both my M2s.
And the lenses had some design problems too in retrospect, and were difficult or near impossible to adapt to another camera. I think this camera WAS the reason why Zeiss gave up on the camera business. It's a camera you can only love till you use it. And I agree about the interchangeable backs. Great idea, lousy execution.
And then there was Yashica/Kyocera. The RTS I was a nightmare for its electronics, but they got it more right in the subsequent cameras. The RTS's user interface was a dream, it had the same feeling of quality and the same superb lenses that the Contarex had, only the contrast was better. Delightful to use, marvelous interface, superb quality in both body and lens. The RTS III is the Contarex Bullseye done right.