I have the 18-200mm Nikkor. I have to say, it is a great lens if you know what you are getting into. As people have pointed out, it is never going to be as good as a dedicated lens. I had some reservations about buying it after reading the dpreview. They are quite critical of it (with just cause). But you have to remember what this lens is being compared to. If you are going to compare it to other telephoto lenses with less diverse ranges than this one then yes, it is going to lose out. However, if you are like me and dont want to constantly change lenses, risk dust getting in, and don't want to miss a moment because you need to swap out lenses then this one is for you. Dpreview is particularly critical about this lenses performance around 135mm. In my experience their review somewhat exaggerates the problem by (as is dpreviews way) putting the lens into extreme situations. I have had some fantastic results with it without major gripes about the sharpness of the lens. Also, I haven't noticed the distortion at 18mm being as bad as it was made out in the review...but perhaps that's just me.
I was seriously considering the 18-200mm Sigma lens, but the Nikkor is a bit faster, has the proper CPU inputs for the Nikon bodies and has the VRII which, in my experience, works fantastically. This lense is a good bit heavier than the Sigma and it makes my little D60 feel a little front-heavy, but with the way you place your hands under the lens it really makes no difference anyway. You do pay for all of this, I have to say, but I haven't regretted it at all...