My reaction to every fast lens Sigma release seems to be "I can't believe something this good costs less than $1,000." A new favorite -- the 50mm f1.4 DG HSM -- joins Sigma's Art series of lenses, which already includes two of my all-time sub-$1K favorites, the 35mm f1.4 and 18-35mm f1.8. Designed to work on full-frame cameras (but, as always, compatible with APS-C models), the 50mm f1.4 has a premium-feeling build quality and excellent tension and tactile feedback on its focus ring, and it produces sharp, bright images with little distortion. There's some softness at its narrowest aperture, f16, and off-angle wide open it has a tendency toward chromatic aberration, but these are typical problems. (Caveats: I had only a day and a half to test the lens, and due to renovations in our office I could not run my usual array of tests. If you're looking for test geekery before you buy, I suggest you check out Imaging Resources' results.)
A note about the image samples: all are raw files with no luminance noise compression, which blurs the photo slightly, so that the most detail possible is preserved. As a result, your mileage may vary.