Summary: To start, I tested this lens on a Nikon D800 body, ISO 200, Induro Tripod with PHQ-1 Pan Head, and remote shutter release with mirror up. I just received this lens yesterday from Amazon. I shot at f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22 on aperture priority. Focal lengths used were 70mm 100mm 200mm. I used both manual and auto-focus modes. All RAW files were viewed at 100% in Photoshop CS6 after performing lens correction with Photoshop.
Summary: It's a GOOD LENS for the PRICE.... HOWEVER, Autofocusing isn't perfect. If you're a photographer who shoots alot of sports events, and you have to rely on using Autofocusing most of the time... this isn't the lens for you. It tends to focus in front of the subject and not "ON" the subject. If you can find a used Canon 70-200mm... get it. Also you're GOING TO NEED A MONOPOD because it's a heavy lens, and hard to hold still.
Summary: This lens is built really well and looks sweet on your camera but the performance is not up to par with the price tag. The main problem I had was the quality of the pictures, most of them would come out fairly soft even if it was on a tripod. I would look elsewhere if you are looking for a great macro lens.
Summary: I ended up buying this lens used on eBay. I did get it used, initially, from Amazon, fulfilled by Amazon, and said to be in great condition. I got the lens, put it on my camera, and it would not focus. Just a "ugn, ugn ugn" sound and no change in focus. Also, I had to force the zoom ring with both hands for it to budge. I sent it back immediately. I doubt I will buy used from Amazon ever again. As far as the lens goes, it's decent.
Summary: At this point, almost every lens I own is a Sigma, the only exception being my Tamron 17-50 2.8 (which is a very, very solid lens). I've owned the Sigma 120-300 2.8 for a handful of days (the best lens I've used), and the Nikkor 70-200 2.8 VR. Both were returned due to the cost that I simply couldn't stomach at this point. I had read the professional reviews for this lens, and was worried about purchasing it because they weren't exactly glowing.
Summary: Couldn't wait for it to come in the mail. Sigma is a good brand, but I got a 'lemon' :( It was soft at many different apertures; I just couldn't believe it. I took many pictures in many different lighting conditions, and all were quite soft. My current lens works great (the lens that came with the camera) and gives me very sharp results so I expected the same out of this lens.