Well, I have an interesting experience to share with you. I own a Nikkor 18-200mm lens. When mounted on my D-40 it was an OK lens, but no great shakes.
I have since, upgraded to the Nikon D-5000 camera, (which arrived today) and when I mounted the Nikkor 18-200mm lens on the D-5000, it was like a night and day difference. The lens is needle sharp and right on the mark. I was amazed and my opinion of the Nikkor 18-200mm lens changed immediately. So IMO the performance of that lens has a great deal to do with what Nikon camera you mount it on!
Today I love the Nikkor 18-200mm lens. One week ago I viewed it as a so-so lens. Wow! what a difference.