I have been pretty happy with my Sigma 20mm f/1.8. It's bright, it's wide and it's sharp... or is it? I've had it for a few years, and been very pleased with its performance, but after I have started using it on the full frame D700, I have realized that it's quite fuzzy in the corners even stopped down. In most cases I haven't noticed, but recently I started thinking that my usually very steady hands couldn't hold the camera still anymore, because most of my images from a certain period came out blurred.
I looked through my images in Lightroom and had to admit that most of the 20mm images were quite blurred in the corners and some with very discrete dark on bright details where definitely unsharp.
Just today I shot using a Magic Arm on a rail and took a short series with the 20mm followed by some similar shots with my Sigma 50mm f/1.4. The 50mm was orders of a magnitude sharper! I suspect the 20mm might be misaligned, because the problem seems to be worse in the upper right hand corner, but it's not tack sharp anywhere but in the center.
I'll be more careful using the 20 in the future and have already started looking at the new Nikon 20mm f/1.8, which is probably way sharper. I love the bright 20mm on full frame. A wideangle just after my heart.