Bryan at The-Digital-Picture has completed his review of Sigma's unique 20mm f/1.4 DG Art series lens. This is the widest f/1.4 lens you can get for your full frame DSLR.
As always with third party lenses on Canon cameras, there is some autofocus inconsistencies that may arise. However, your experience may differ from the reviewers depending on what your style of shooting is.
From TDP:
Invite the sun to set and turn down the lights – this lens has no fear of the dark. As of review time, the Sigma 20mm f/1.4 DG HSM Art Lens has a wider aperture than any lens wider than 24mm. With this unique tool in your kit, freeze action in low light situations calling for a wide angle of view.
I can't discount some AF inconsistency encountered, but this lens overall is a very good one. The lens design, the build quality, the image quality and the price all come together to make the 20mm f/1.4 DG HSM Art Lens another great addition to the Art lens lineup for Sigma. Read the full review