I picked up a used Canon EF 8-15mm f/4L USM Fisheye lens and had my first chance to try it out this afternoon. I made a quick trip to two locations, Oakwood Cemetery and Waterford. On a full frame camera, the lens gives you a 180 degree diagonal fisheye image at 14mm and a nearly 180 degree diagonal field of view at 15mm. At 8mm, the lens gives you a 180 degree circular fisheye image.
First, are two images from Oakwood Cemetery at 8mm and 15mm from the same shooting location.
Here are two circular fisheye images from Peebles Island and Waterford Harbor followed by the same Waterford Harbor scene at 15mm and 14mm.