I got a Diana 20mm fisheye lens and adapter recently and I haven't done much with it but take test shots. When I first got it I noticed it wasn't anywhere near as "dreamy" as I thought it would be, but it's still plenty soft.I created a vignette by taking the lens apart and putting an f/5.6 Lensbaby aperture disk behind the front lens element. It works okay in some cases but in others its rather extreme and awkward. I like it done in-camera much better than in PS, anyway. I also found that if you flip the front element backwards and put it back in it gives you a lot more blurring around the edges, like a Lensbaby, but I have no need for that.
The 20mm lens works out to be a normal lens on the 40D, even though it shouldn't be. It should be a 32mm equivalent according to the 1.6x factor, but it gives the same field of view as my 28mm prime, which is close to a normal lens on APS-C. I don't know what the problem is. There are some weird flare issues around strong light sources, giving almost a complete ring in some cases, like in the night shot above. Barrel distortion is very strong, too.
I drilled out the hole in the adapter because it was smaller than the rear element of the lens, which serves no purpose other than to take away about a stop of light, in a lens which is already very slow. I think it yielded a little extra blur, too. Maybe.
I have nothing else to say about this.