This Sigma DC 18-200mm OS HSM lens I am using with my Nikon D40. The inclusion of HSM (Hyper-Sonic Motor) enables auto-focusing without support from any mechanics on the body. This usually takes less than a second to complete and makes a slightly louder sound than the Nikkor 18-200mm VR lens.
This is a great lens but two minor points hold it back- firstly it is rather heavy (unsurprisingly for the zoom range), and secondly the zoom-wheel turns the opposite way (anti-clockwise to zoom in) as to what I am used to.
Minor quibbles aside as said this is a great lens. The OS (Optical Stabilisation) works very well at long focal lengths but does drain battery life slightly, a worthy price for non-blurry pictures.
On the downside this lens is f6.3 at the long end (As opposed to the Nikkor 18-200 which is f5.6), resulting in longer exposures needed to allow more light to enter the lens. There is also some vignetting at the 150-200mm range of this lens, which can be quite apparent on sunny days. The relatively low speed however is counteracted by the OS system which works pretty well to alleviate camera shake from the longer exposures required.
Personal preference had me pick this over the Tamron 18-250mm lens which is slightly cheaper but does not feature optical stabilisation. The Nikkor 18-200mm VR model is also a great lens which overshadows this one, but with the Nikkor being almost £200 more this offering from Sigma is a wonderful, if somewhat heavy general purpose lens.