I have the Sigma Art 35, and I honestly love it. Sharp, bright, and lovely bokeh and colors. I do believe this lens will be, if not totally equal to the $4k+ Zeiss lens in sharpness, etc., then at least 95% there, based on the fact Sigma seems to be vouching the lens will be even better than the 35.
I also think the only reason Zeiss does not put AF on their nice lenses, which even the cheap-o kit lenses everyone else makes do include, is that they only have the ability to make so-so AF. They only put it on a few cheap-o Sony lenses, and it is only cheap-o AF. They make MF lenses with glass that works like a decent precision microscope, but they simply have nothing special to offer in the area of AF (OR I.S.), so they shy away from trying, so they don't catch the flak for slow/inaccurate/mediocre AF that would tarnish the reputation of their optics.