Very interesting fisheye concept.
My fisheye is the 15mm, 180-degree diagonal fisheye. It won't do a circle, but it covers 180-degrees on the diagonal. I was really interested in the 8=15mm, but I wanted to be able to de-fish some of the images, as I do around 90% of the time now.
It's great for big-sky shots, like this:
Storm warning... by
dcstep, on Flickr
Dave