I think it all depends on how you use it.
mine is good to me:
DSG_1123_Sigma_corner sharpness test crop..jpg
A corner crop(5mm from the very edge of the top RH corner) cropped at 100% view, saved from 1038 down to 1024 pixels, and then saved at 75% quality to keep within the 250kb limit.
Aperture used was something like f/20, so some diffraction is evident, and I think approx f/8 yields about the best sharpness/contrast detail, if absolute corner performance is paramount.
no in camera sharpening done(as it's too coarse, or unrefined) so USM is used in PP.
That involves; USM figures of 55,4,4 in CaptureNX2's USM tool.. how that translates to other software is beyond my level of understaning.
This small patch of grass part of the scene is basically insignificant in overall terms to the entire scene tho.
Whilst the IQ is not in the
full on professional league, for a $400 lens, it's bloody damned good to freaking excellent!!
This part of the grass is about 40cm from the front element, and I think with this lens not exactly ideal, as it's appears to produce better sharpness results when the subject is a little further away.