The new Sigma 18-35 1.8 is the sharpest zoom for APS-C, concluded DXO when they published their test for the Sigma 18-35. DPReview, a DXO affiliate, also posted similarly glowing reviews of the Sigma 18-35, giving it their gold award. When the Sigma 18-35 was announced back in April, I was drooling at the 1.8 aperture (the widest of any zoom), and was cautiously optimistic based on the MTF charts. I'm surprised that according to DPR, the Sigma 18-35 is actually almost as sharp as the Sigma 35 1.4 at 35mm, the least sharp focal length of the Sigma 18-35. Wow.
If I had only an APS-C camera, I would probably get the Sigma 18-35 as my standard zoom - for the sharpness as much as the shallow depth of field. However, if you have a full frame and APS-C camera, it is a little less attractive. Its DOF is somewhat similar to a 24-50 f/2.8 full frame lens, and I personally would prefer a Nikon 24-70 2.8, Nikon 28-70 2.8 or Sigma 35 1.4 over a 24-50 2.8. The first two lenses have more versatility (70mm can be used for portraits without needing to be too close and distorting the subject's face). As for the Sigma 35 1.4, it has a shallower DOF than the 24-50 2.8. Nonetheless, the Sigma 18-35 should be useful for a lot of photographers who take pictures of people.
One thing I found interesting in DPR's review of the Sigma 18-35 is that the autofocus can at times be inaccurate. That is the same criticism I have of the Sigma 35 1.4. On the Nikon D600, I find the best results on the Sigma 35 1.4 by using AF-C with dynamic 9-point (not dynamic 51) but I still have to take a few shots to make sure at least one is critically sharp. (I have the Sigma USB dock but haven't used it.) Alternatively, you can use contrast detection (using live view).