I notice that there is a wide dicotomy between the positive reviews and the negative ones.
It's difficult to see what people are expecting.
Sigma's QA has always been in question. I also wonder, therefore, if that it's partly responsible for some people saying that this lens is sharp wide open, and at 400, while others say that it is not sharp open, and at 400.
Too bad we may never know. The small photo's posted can't give us a good idea about that.