I was just shooting casual shots with it today on the RD-1, where it makes a FL closer to 25/24 than 27. It vignettes heavily on that, but on a film camera (ZI) barely much at all.
I don't have either of the CV lenses you mention to compare it to, so I'm not much help comparison-wise. I really like the Zeiss look and the way the ZM lenses handle, so I replaced all of my CV glass with Zeiss. But the CV lenses certainly have unique advantages, too.
I don't have either of the ZM 21s, either, but there are plenty of photos on Tom's flickr site.
I'm very happy with the 18/4.