this is taken from kenrockwell.com full review
http://www.kenrockwell.com/canon/lenses/18-200mm.htm
Zooming
Zooming is good, which is important for a lens that zooms this far.
There is so much moving around as you zoom that it's more difficult to zoom if you point the lens directly up or down, since you now have to pull half the lens up or down with the zoom ring. If this is difficult for you, point the lens the other way first if it helps you zoom in (pointed down) or zoom out (point it up). It's no problem pointed horizontally.
The range is a bit cramped at the shorter end, making it less easy to set precise framing. It's fine throughout the rest of the range.
The lens will extend when carried on a camera around your neck and it points down. There is a lock to keep the lens set to 18mm, but it's as much trouble to set ad release this lock as it it to have to rack the zoom back in as you bring the camera to your eye.
At infinity, 200mm really is 200mm.
At closer distances, like 20 feet and closer, 200mm isn't really 200mm. At close range, you'll see almost no change as you zoom between 100mm and 200mm.
This is normal for ultra-range zooms that focus as closely as the 18-200mm. Canon and Nikon both play optical tricks to allow long lenses to focus this close. Those tricks are that they reduce the focal length as yo focus more closely, which is how they get these lenses to focus this close. You probably don't realize this, but physics tells us that it is impossible to focus a true 200mm lens any closer than 800mm (0.8 meters or 2.6 feet). This Canon 18-200 focuses to 1.5 feet or 0.45m!