The Sigma 35mm F1.4 DG HSM Art isn't the only third-party option in town. The Tamron SP 35mm f/1.8 has its own set of benefits, including a lower price tag, more uniform illumination, closer focus, and image stabilization.
Very sharp; Wide aperture; Attractive price; Available for multiple camera systems; Solid build quality; Eligible for mount conversion
Heavy vignette effect; Omits image stabilization; Not the best option for ultra-high-resolution cameras