Summary: The lens is all right for the price. I had this on a D7000 and the quality is not even close to the Nikon 35mm 1.8 lenses. It is a decent and cheap walk around lens. The "OS" works relatively good, focus is pretty accurate. Not much hunting. Speed is ok, not super quick. The HSM is a bit noisy in video play back mode but how often do you use your DSLR camera to capture video anyway. It might be loud when you take pictures in a quiet place like churches or museums.
Summary: Ill make this short and sweet. I got this because I wanted a kit lens replacement. It does that relatively well. Being able to stop to 2.8 is nice. The colors are pretty accurate and the sharpness is ok in the center. My one major gripe with this lens is the barrel distortion. Wide open at 18 mill. is a mess. Everything is wobbly and bendy. It straightens up as you zoom in.
Summary: I bought this lens 1.5 years ago, and it was my main lens. Used it a lot, traveled a lot with it. but suddenly my camera (nikon d90) started to show the "F--" error and now this lens stopped to work. Sad I live in brazil and I forgot to register mine for the extended warranty.
Summary: Been using this lens for about a week now. Overall performance is Ok. The photos come out fairly sharp. However, when I switch over to my canon 50mm 1.8, the sharpness is way better than this lens. For more than 3x the cost, I would expect it to be close comparison, but I definitely love the sharpness of my 50mm a lot more. The other thing is that there is no IS on this.
Summary: I really wanted to buy this lens. Unfortunately, my kit lens (Nikon 18-55 EDII) is a lot sharper than this!! I tried this previously at a local trade show. Although the build quality seems very good. It appears very soft at f/2.8 across its entire zoom range. That in itself was a deal breaker for me. The whole idea of having a lens like this is low light performance and some respectable sharpness. Sharpness does not come close to respectable for my taste.
Summary: I've owned Sigma before and this is yet another lens from them that has the front focus problem. I used this on the D200. I own the Sigma 24 1.8 and I had to send both the lens and camera body to Sigma to fix the focusing problem. I just don't want to bother with that any more so I returned it. The Sigma 18-50 has the potential to be a good lens but focusing issue causes images to be extremely soft (especially at 50mm).
Summary: I've bot this lens and was soon disappointed. With all the great value (good zoom range, image quality and 2.8 aperture over the whole range for the price) the lens suffered from terrible front focus on my Digital Rebel XT. Interestingly enough, my friend's Canon EOS 30D didn't have the problem. Also I bought the lens through a third-party vendor (47th street photo) on Amazon and hence had troubles reaching them for a return or refund.
Summary: A very good value for the modest investment. My copy is not without an issue however. I was hoping for uniform edge to edge sharpness throughout the 18mm image with my Canon 30D. Unfortunately at 18mm it is not usable at f2.8 to f5.6 without considerable edge blurring within 10-15% of the image border. This influence is all around the image and represents a large percentage of the overall image. I will avoid this f stop range at 18mm.