So the lens arrived 4 days ago I could play a little bit. Nothing serious till now, just the usual tests. Fortunately nothing bad that's told about it is true - at least not on my lens.
Build is good - there is some wobbling in the barrel but it sounds heavy - so it's ok.
Aperture blades close down perfectly - as good as the 70-200 and clearly better than my 16-50 and leaps better than the older Sigma 1.8/28.
The focusing system is unusual but I can get around with out. However the lens hood is way too short - I can hold the hood of the 70-200 in front of it and it still doesn't vignette. Even with the long hood there are still strong flares visible - but again better than with the older Sigma.
Also of interest but not fully tested yet is sharpness/resolution. It seems to be on par with the older Sigma - which is great.
Oh, and it focuses very (very) close.
Sharpness test pictures at 20cm at f/1.8, f/2.8 and f/4