Optically the adapter is pretty lousy. You can see the poor quality just by peering through the viewfinder - it gets extremely soft and blurry around the edges and there’s a lot of uncorrected chromatic aberration. I tried shooting at 2.8 to see what it looked like, and it was pretty awful - you really want to stop down as far as you can go with this. The same goes for the removable single-element macro ring - you can use it for cheap macro photography, but with very narrow depth of field and a lot of distortion.
On a 28mm lens the adapter projects nearly a full circle onto the negative, with the characteristic bowl-shaped fisheye look. With longer focal lengths - 40, 50mm - you get an image that fills the whole negative. I’ve tried it with my Canon 28-105 zoom for flexibility, though I think I’ll generally use it with my 28mm prime, in order to squeeze what little image quality I can get out of the thing.
As with all super wide-angle lenses (or facsimiles thereof) the close focusing is really close. There’s a huge depth of field.
Note that if you use the adapter with a telephoto lens - say 105mm - you lose the fisheye effect and just get a soft image in the centre with a lot of heavy blurring and stretching as you near the edges. Which can be interesting if you want to make photos from your expensive Canon EOS camera look sort of like those produced by a Diana plastic camera, I suppose.
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