This is going to be a little untypical review, as I am using this lens in A (ex Minolta, now Sony, mount) with a motorized adapter, on a Sony NEX 7, and soon on a FF Sony A7.
The lens is very unbalanced on the NEX body, which I am used too, using many other DLSR optics on it, so not an issue for me. Just carry and operate the combo holding the lens, close to the adapter mount, and all is fine. The combo looks ugly, impressive, and certainly means business. It screams "serious gear".
One thing I am totally in love with about high end Sigma lenses is the HSM motor. It is virtually silent and very, very quick. It gives excellent performance on the NEX 7 body via the LA-EA2 adapter. DSLR like performance.
The build quality is good, the matte EX finish tends to wear off with time, but so far this unit I bought used is in mint cosmetic condition. The front element is not as large or gorgeous as on the 70-200/2.8 HSM (my favorite AF lens on Earth) but it is adequate for the size of this baby. The lens is a monster, 82mm filter thread is not ordinary, but it's way shorter than it's 24-70 siblings in other makes, be it CaNikon or even Tamron. It's fat, but short.
The optical quality is just awesome. I used to have several Tamron 28-75/2.8 lenses and they were optically great, but this is clearly above, with a very recognizable high end Sigma signature in both the razor sharpness (yes, even from max aperture in the center) and the gorgeous and unique creamyness of the OOF areas. This stark contrast between sharp and creamy is what I love so in those high end Sigmas. My unit is perfectly calibrated for my NEX 7 without any intervention or adjust on my part, which is great. Overall super impressed, and can't wait to add another 70-200 to my collection.