I have a Nikon D300, and it originally came with the Sigma 18 -200mm lens, which I have successfully used for a couple of years, and with which I have taken many pictures. So why buy one of these relatively expensive Nikon lenses, that mostly does the same job?
Well, the first thing to understand is just how useful a lens with this spread of focal length really is, and I learnt that with the Sigma, and the Nikon lens simply reinforces the opinion. If you only have one zoom-type lens, then the 18-200 range is absolutely fantastic. It changes the way that you look at things.
Down at the 18mm end of the travel, you are looking at a landscape or a building where the image in the viewfinder is probably a lot higher and wider than you actually see in real life. Most of us just use the bit in the middle of our vision for most of the time. At the 18mm end, you need to look around a lot more at what you want to capture, and then you need to look around in the viewfinder a lot, to make sure you have framed the shot you want. And then you want the detail, so you head towards the other end of the zoom, towards the 200mm setting, and there is the detail you want to capture. You can go, with the twist of your wrist, from the whole of your view, to a small piece of the detail. None of us routinely see the world this way, so it takes a while to realise what such a lens does. The only other thing that does that, is a one litre sports motorbike, and on one of those you will risk being locked up if you are caught twisting your wrist that much!
If you want a beautiful bokeh with the detail, that seems to be the Nikon's biggest advantage over the Sigma, so far as I can see. It also feels a lot more solid, but then the Sigma has survived a couple of years of use, and the Nikon is still new, so time alone will tell. As it is, the only disadvantage to having an 18-200mm lens is that you loose a bit of the perceived credibility that comes with lots of strapping-on of different lenses, for different shots. Of course, with the single lens, you capture things you would otherwise have missed while doing the lens change, but the reality is that with a lens of this type, you really can use your digital SLR camera like one of those drunk ladies camera's, that sometimes make you wonder why you bother lugging a big kit about! These lenses really are that good, that you will find that you fit it, and forget it, for most of the time.
When I go out to try to do some serious wildlife pictures, I will always take (OK lug), the 80-400mm zoom but I will walk in with the 18-200 fitted and capture what I can on the way. Sometimes specialist is good, sometimes it's versatility that delivers, and for versatility, this lens simply delivers what you need, for a huge percentage of both time and subject.