The 18-200 is poor. It has a very long range but that's it. It has worse sharpness CA vignetting and massive distortion at the wide angle compared to the above listed much cheaper lenses. It suffers from lens creep, but at least it has VR, thank god too I mean there must be something in this to even remotely justify the cost.
Respectfully, I have a different opinion as an owner and long-time user of this lens.
We like our 18-200. I has no lens creep whatever (after 6 months of use and 30K+ pictures taken with it in temps from well below freezing to well above "hotter than heck"), but that is not to say that others in the line don't... only that the one that we own doesn't creep.
I don't have any idea how it would perform on shooting test patterns. I have never felt an artistic need to actually shoot a test pattern, since really they are kind of boring.... so I don't know anything about the distortions. I have never shot a brick wall in my life. Again, they are boring, and I really have no idea why anybody would want to.
You can use it to take good pictures, and carry one lens instead of 3. It is sharp, behaves well and the VR works exceptionally well.
I am not just being one of "those guys" who has to defend previous purchases. See my comments about my 105 VR Macro in other posts to support this.
The 18-200 VR is an everyday working lens for us, and we take a lot of pictures. Whether those pictures are of adequate quality is for y'all to decide.
Having said all of this, if I were in the original poster's position it would not be my next lens purchase. It is a "jacknife" lens, and an expensive one at that. If I were the original poster, I would keep the kit lens and get a 55-200 VR for cheap, and then go out and use the dog-snot out of my equipment... upgrading when the equipment becomes the limiting factor, and selling the D40 with the two lenses as a "kit", upgrading to the D80 (or the D90 by that time) or the D300 in a year or so.
No, I am not saying that the 18-55 Kit and the inexpensive 55-200 VR lenses are "the best money can buy", far from it. But you can take good pictures with them (the limiting factor is generally always the skill of the photographer, not the equipment... if this wasn't true, all you would have to do to be the second coming of Ansel Adams is to buy the most expensive equipment), and they are inexpensive. IMHO the 55-200 is a great lens "for the money"... keeping in mind that it is a slow, plastic consumer grade lens... but it is also just a few dollars more money than a Big Mac and a jumbo order of fries.