The 15mm f/2.8 is a full-frame fisheye lens, not a circular fisheye lens., which means it provides 180° angle of view only across the diagonal – not circular. I have found it to be a great lens. I’ve had both the Canon and Sigma. I’ve never owned both at the same time to compare side by side on the same settings. However, I believe there is very little difference between the two. I would consider the Sigma maybe having a very slight edge on IQ and the Canon better in build quality. Reason being that the finish on the Sigma has started to peel over the years.

On the full frame camera you get a pronounced yet pleasing fisheye effect.

On the 1.3 crop cameras (Canon 1D series) the fisheye is less pronounced and yields a pleasing perspective on wide angle landscapes.

On the 1.6 crop cameras the FE is a sharp and economical solution for ultra wide angle needs – equivalent to about 17mm perspective on a 35mm camera (the multiplication factor by 1.6 does not apply for fisheyes). If the horizon is anywhere near center of frame, its hard to determine any fish-eye effect at all. In fact it many times looks more natural than the slant out look of the rectilinear.

I’m posting examples for all three camera types – the full-frame, 1.3 and 1.6 crops.