Over the past 6 months I have shot extensively with the Sigma Sd Quattro and H model and my conclusion is that it may be my favorite system to pull amazings shots with. It has its struggles one of which I have found to be microblurring but that is more a fault of my own then of the camera. It needs a tripod and or a steady steady hand. Usually I am able to provide a steady hand but when walking around with gear and a 4 camera setup it starts to get to you.
It's purpose was never street or urban portraits but I have pushed it successfully many times and when I do things right it delivers like a medium format camera.
I have used it quite abit in the portrait realm and for slow steady shootings it's really a beast. It has taken quite a few shots alot of people have really loved and with the Sigma's largest yet apsh sized sensor, the bokeh and depth of field have both been radically improved.