Sigma has always been a discount brand. A cheaper alternative to the big name lenses from Canon and Nikon. Over the last decade though, they've started filling in niches where the big guys have left gaps. An excellent example is the 30mm f/1.4 EX DC, a lens that gave a field of view similar to 50mm on cropped sensor cameras with a wide aperture. This wasn't the best performing lens, but at the price it was a great one to have. The 10-20mm was another example. In the last few years though, Sigma is attacking the big guys in the high end. The latest camera to do this is the 50mm f/1.4 DG HSM Art.
review of this lens is over at SLR Gear.
We now have a price, $1030 USD. Amazing for a lens of this level. Source