I had to replace my stolen Nikon 18-200mm f/3.5-5.6G AF-S ED VR with the Sigma AF 18-200mm f/3.5-6.3 DC OS (Optical Stabilizer) Zoom Lens. I was first a little skeptical because of the Sigma lens costing half that of the Nikon replacement (the old saying you get what you paid for).
But I can say without a doubt, The Sigma lens is equal in quality and perfomance at half the price.