I also have the 21/4 and have used it a bit on several different bodies. No complaints; its quality may not be the equal of Leica/Zeiss in a laboratory environment, but nothing I've seen makes me think that those differences are important for my uses. I just looked through some photos for which I've used it (http://www.flickr.com/search/?w=67439103@N00&q=21%2F4) and I see vignetting in some of them, but nothing to worry about. (The ones where it's strongest are in infrared, where vignetting is always exaggerated.)

I think it's safe to say that the quality of the lens is more than good enough for exploring the focal length.