Hi Martin
Welcome to the forum. I would recommend having a look at the site
www.slrgear.com where they have tested quite a range of the main manufacturers lenses and some of the independents. For the lenses they have tested there are interactive graphs showing the sharpness, chromatic aberation, vignetting and barrel/pincusion distortion for the lenses.
The
18-200 VR that you are considering does seem to exhibit considerable edge softness in the mid range until stopped down a bit. Compare it with the
18-70 which although doesn't have the zoom range or VR appears to have superior performance, is considerably cheaper and if often offered as a kit with Nikon bodies. If you want a bit more zoom range, as an alternative you might want to consider the
18-135 which appears to between the two both in terms of price and performance, although it still doesnt have VR
When I was looking for a digital SLR I looked at all these lenses and in the end decided that I wanted the sharpness of the 18-70 but with image stabalisation - remember its not good being able to hand hold the lens if the resulting images are not sharp enough, so I decided the
Canon 18-85 IS was the lens for me and bought a 20D to go with it
