Dustin has completed the Canon Rumors reviews of the Sigma 20mm f/1.4 DG Art. This is the widest f/1.4 prime you can get for your full frame DSLR and will likely give you a unique perspective for your work.
Dustin found this lens to be a mixed bag in a lot of ways. As we've come to expect from Sigma's ART series primes, the optical performance is very good, with a few shortcomings, but the size and lack of weather sealing (again) may detract a lot of photographers from buying this unique lens from Sigma.
If you need great low light performance in a wide angle lens for events or wedding work, it is hard to think of a better alternative. The resolution from the lens is fantastic. The price relative to the build quality and performance is strong, but at the same time I think that Sigma stuck a little too closely to its ART formula for this lens. Yes, it is now the class leader in maximum aperture value at 20mm, but at the cost of being the largest lens in the segment and the only one without the ability to use traditional filters. You can debate about the value of weather sealing, but the reality is that the lack of it will probably send a lot of photographers elsewhere. I also think that Sigma missed the boat by not working harder at controlling coma – they would have sold many of these lenses to astrophotographers. Read the full review
This is definitely a unique lens, and will likely make a niche market of photographers quite happy.