JoJu ... you have qualified as the defender of the faith now ... 
- the field curvature is an issue STARTING at 18mm - not across the range. You are probably aware that we use a slicing approach to determine the best spot on the focus field. If the center and corner slice differ by "5 slices" I wouldn't bother too much - yes, you will see that on a wall but - as mentioned in the review - who really cares. At 24mm f/5.6 the Sigma was at up to 9 slices and at the edge of what we covering (ca. +/-12 slices). The Canon 11-24L didn't have this issue.
- remember that 50mp are magnifying any issues substantially. Yes, I will provide the 21mp figures soon (and that will, as usual, ease the pain)
- a comparison with the Nikkor 14-28mm is meaningless in the Canon section. The benchmark is the 11-24mm L here. The 11-24L flares as well, of course, but not quite as much as the Sigma.
- if your sample is different - good for you - yet this was a brand new lens and passed Sigma's QC. The center was quite fine centering-wise. From there on it is not my problem anymore. PZ does not exist to defend manufacturers. It is worth remembering that any criticism is also meant to tell the manufacturers that they have to push more QC-wise and quality-wise. If you want to read hallelujah ratings ... well, there are many sites out there that do that.
- the field curvature is an issue STARTING at 18mm - not across the range. You are probably aware that we use a slicing approach to determine the best spot on the focus field. If the center and corner slice differ by "5 slices" I wouldn't bother too much - yes, you will see that on a wall but - as mentioned in the review - who really cares. At 24mm f/5.6 the Sigma was at up to 9 slices and at the edge of what we covering (ca. +/-12 slices). The Canon 11-24L didn't have this issue.
- remember that 50mp are magnifying any issues substantially. Yes, I will provide the 21mp figures soon (and that will, as usual, ease the pain)
- a comparison with the Nikkor 14-28mm is meaningless in the Canon section. The benchmark is the 11-24mm L here. The 11-24L flares as well, of course, but not quite as much as the Sigma.
- if your sample is different - good for you - yet this was a brand new lens and passed Sigma's QC. The center was quite fine centering-wise. From there on it is not my problem anymore. PZ does not exist to defend manufacturers. It is worth remembering that any criticism is also meant to tell the manufacturers that they have to push more QC-wise and quality-wise. If you want to read hallelujah ratings ... well, there are many sites out there that do that.