Now to the essence: The perceived quality of this Sigma lens is terrific. Caught you an intact unit, you can look forward. Whether it is because of some quality scattering, of which one so often hears, I can not say. I guess I've had bad luck.
The Focus is very fast, I have not measured it, measured values do not interest me anyway.
The operation of the zoom is pleasantly firm but not stiff.
The weight can be felt, but I use a wrist strap and transporting the camera on the long arm. Together with my 6d I move a package that is not too heavy and makes awesome sharp images, up to the image corners from grill Four!
Now you can discuss everything back and forth. For me is certain: I would not have this lens, I would buy it immediately.
My L lenses is there at least not after.
Any further assessment I leave like my contemporaries who, like me, dare to take this step.
Have lots of fun with it.