I once had the Zenitar and found it excellent.
Compared to the 16mm Zeiss F-Distagon fisheye, which I kept, it was slightly sharper in the center, (very) slightly softer in the corners (these differences only being noticable when nitpicking) and a tiny bit less coverage (if the Zeiss is really 16mm, then the Zenitar might be 16.5 mm). Also, good colours and pretty good flare resistance. In restrospect, I should have kept it.
While it theoretically needs the rear filters to be in place for best performance, I didn't notice any difference without.