Summary: Very sharp, great contrast and color. Absolutely no complaints. A wonderful alternative to the more expensive Nikon equivalent. I had rented the Nikon fisheye previously and could not see a noticeable difference. A fun lens that will give you amazing results.
Excerpt: I am a professional wedding and stock photographer (architecture and travel) and bought this lens to use with my Canon 5D Mark III. I used to have a Tokina 10-17mm Fisheye, but the Tokina is soft on the borders when used with a Full Frame camera (very soft). The Sigma 15mm is tack sharp in the center and very good on the borders. I read in the reviews when I was searching for a fisheye designed for full frame cameras that this Sigma 15mm has a slow autofocus.
Pros: Durable, Lightweight, Nice Bokeh, Sharp, Strong Construction
Excerpt: I already use a Canon 17-40 mm lens on Canon 5d III but to go even wider, the Sigma 15 mm was needed. A well made solid lens. Noisy, slow focus but this is not an issue with how the lens is typically used. Images at f8 and f11 are very sharp. Yes lots of distortion but it is a 180 degree view lens on a full frame body. Amazing depth of field particularly at f16 and f22.
Pros: Amazing depth of field, Ultra Wide Angle View
Summary: This is now my favorite lens in my arsenal. It focuses at incredible speed on my D800 and the images are just stunning. I usually prefer Nikon version lenses, however the current Nikon is dated and Sigma has really raised the bar. I would highly encourage trying this lens out.
Summary: I love how wide this lens is, I can stretch my arm to my wide and see my hand in the frame. And having auto focus is awesome for those low angle shots that dont allow you to look through the viewfinder.