Even at the back of the tunnel I have it mounted in, it does seem to cover. Looking through the corners of my GG I see a lozenge of clear lens of about 1/3 of the full glass, so illumination will be there but less. Performance goes down as you get larger than 5x7, but not as quickly as a simple close-up lens does, and there's an amount of distortion that gets worse, the larger the film size but isn't noticible on 4x5.
I would call it "coverage" but I wouldn't call it "good". :-)
Here's a quick view through my 8x10 camera. Don't take the sharpness as shown here seriously--just the coverage and distortion:
achromat-on-8x10
by Michael Darnton, on Flickr
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