Looking at a lens

[soydios]

Allow me to throw in my 2 cents as an owner of the Nikkor 18-200mm AF-S VR. Excerpted from my Nikon Lens Guide:

AF-S 18-200mm f/3.5-5.6G DX VR $680
This is a full AF-S lens, and is most often bundled with the D200 or D300, but is also available for purchase separately. It has the same build quality as the 18-70mm, but obviously has a whopping 11.1x zoom range, from moderate wideangle to long telephoto (27-300mm equivalent). Some samples get soft beyond 100mm, and almost all samples have a problem with zoom creep. Because there is so much weighty glass in this lens (16 elements in 12 groups), the zoom barrel will retract or extend when pointed up or down, respectively.
DPReview.com has an excellent summary of all the pros and cons of this lens in their lens review.
Hands-On Impressions: This lens is immensely useful. The optical quality is not perfect anywhere, but this lens is good enough at everything to stay on my camera for 99% of my everyday photos. Autofocus is quick, and my copy is sharp throughout its range in the center of the image. Finally, on the topic of lens creep, it?s only a problem if you point the lens up or down when on a tripod, or if you dangle the camera around your neck.

In short, it has the same optical performance (sharpness, contrast, saturation, chromatic aberrations, distortion, corner light falloff, etc.) as the 18-55mm VR and 55-200mm VR lenses, but with faster true AF-S focusing with instant manual override, and more size, weight, and expense.
When I need to take only one lens with me somewhere on my D50, I take the 18-200mm VR. It's good enough for 99% of my everyday snapshots.
However, for serious photography and/or low-light use, I would get something else. Everyone here seems to recommend the Tamron 17-50mm f/2.8 to fill that role, though I don't have any hands-on experience with it (yet).
A word of experience: keep your 18-55mm kit lens. The 18-200mm VR is a big honking lens, and there are situations (such as parties, tourism, etc.) where the smaller, lighter-weight, and less intimidating 18-55mm will come in handy.