I had the Sigma 20mm f/1.8 for exactly one week. In that time, it missed focus, was slow to focus, and was only acceptably sharp at f/4 or greater. Also, it's huge (82mm filter threads) and has a terrible clutch system toggling between AF and MF where you have to switch BOTH the camera AND the lens in order to toggle between them. If the camera was set to MF and the lens to AF, you couldn't focus at all.
I returned it and replaced it with a Nikon 20mm f/2.8 that I could use wide open in a pinch and fit in my pocket. It was sharper across the frame, focused more accurately, and small enough to take anywhere.