I am a Nikkor lens user for 35 years and it took me almost 6 months to decide wether to buy this Sigma, the Tokina 12-24 or the Nikon 12-24. I was decided for the Sigma mainly for the 3mm difference in the shortest focal length compared with the other two - and 3mm at the shortest focal length is something to take in consideration, at least for myself. The Nikon was very early discarded from the competition as its price is way out of my boundaries, and the Tokina because its sensitiveness to flare. So, first things first.
1 - The build quality of the Sigma is really good. Sturdy, not excessively heavy nor light, it gives a very good balance with the D200. Focus is silent and precise, zoom ring very well damped, its a pleasure to work with it. With a protective filter in front . in my case an UV - the lens cap, although a bit flimsy, is easily handled in and out with the lens hood in its place. really. From my side, I have no problems although it could be a better lens cap. I have compared the "outer" construction with the Nikkor and I really prefer the Sigma.
2 - Image quality " in the field " is really and surprisingly very good to excellent at any aperture -being that at f/8 is really outstanding - or focal length, across all the frame. This is what gave me the biggest surprise. This lens is really sharp. The distortions are well controled - it has the normal distortions for a 10-20 mm focal lens - being that at 20 mm I can notice a slight pincushion distortion. Neverteless, everything is well corrected with PTLens. So no problem. For "normal" shooting, camera balance it is a must but we can dare to be more radical and get excellent "radical" compos. Colour rendition is very good - although my work is 98% black and white. Flare control is amazing. I can shoot with the sun in front of me without getting any flare at all. I am impressed.
I was a little affraid on what concerns the problems some users got with their first sample having some image issues due to some bad centering of the lens elements. It seems I got a very good sample of this baby and it seems that those problems are solved for good nowadays.
For the time being, its the Sigma that is attached most of the time to my D200. It's an hell of an excellent performer.
I am really pleased with my buying and I do not regret it at all. I only can recommend it with the highest rating