Summary: Good value. I received this lens recently, and slapped it on my Nikon D90. Focus is accurate, fast, and quiet at most zoom settings. The weakness there being at the long end you are better off switching to manual focus, not that its horrible, but if you're looking for accuracy your eye will do better. The OS works phenomenally, well above my expectations.
Summary: Like most people, I bought this lens to replace the 18-55 and 55-200mm lenses, which I'd been using for years. As a replacement for those two lenses, I have to say the Sigma has been excellent. I would definitely recommend it. Were cost no object, however, I probably would have gone for the expensive Nikon 18-300mm lens. At the time I bought this one, it was almost triple the price.
Surprisingly good quality for a lens with this zoom range and price point.
P. Jackson, Amazon
3 November 2013
Summary: I am using this lens on a Canon 70D so all comments are made with respect to this body. I have taken about 1000 pictures using the lens on a couple of trips. This lens has an excellent focal length range and is a very useful default lens to leave on the camera. It is a little slow at the 250 mm end of the range, but the camera had no trouble with focusing (typically Canon need at least f5.6 to focus accurately).
Summary: I'm a photography tour leader, and have owned 18-200s made by Sigma, Canon and Tamron. In that same order is how I rate each of the brands with Sigma first. Unfortunately some folk would rather change lenses constantly than use what they perceive to be a lens that is inferior optically. In some cases this is true, but this Sigma 18-250 is the sharpest short to long zoom I've ever owned, and on some of our tours it's the only lens I use.
Summary: As much as it would be to have the Nikon lens this is an acceptable alternative. I have several Sigma lenses and unless I blow things way up I don't see a degradation of quality. Yes Nikon glass would desirable if I could afford them.
Summary: This lens is excellent for the money. There are certainly better lens available, but at a much higher price. This lens is very versatile and is exactly what I wanted. The only downside is on the website, when ordering, they show this lens being ordered with the 72mm UV filter. The 72mm filter will NOT fit this lens!