I got myself the Canon 18-200mm lens as a replacement for the 18-135mm lens that was part of the EOS 550D kit I bought a few months ago. I wanted to expand the range of the zoom because sometimes I would get a bit frustrated when trying to zoom in on details or far objects and the 135mm just wasn't enough to satisfy my eye.
First impressions: tough built, heavier than the 135mm but somehow feels less bulky, fills the hand (is a wider 72mm).
The first time you try to zoom in you have to remember that it has a lock in place that you need to unlock (lower right side of the lens), which I find great for stopping the lens extending itself when I put it in the bag or take it out of it. That used to happen to me all the time with the 135.
Also for the first few days zooming in and out will require a bit more of a forceful action, until the zoom gets more "used to it" and the movement will get quicker.
The zooming, even then, is always perfectly fluid though.
The images I got so far are of daylight photography, I haven't tested it at night yet.
The results are stunning for a lens with this price tag: crisp images, handles the light perfectly, I have noticed no chromatic aberrations at any focal length or aperture. The bokeh is very pleasant to the eye.
The Image Stabiliser works wonders at the 200mm end of the zoom, taking sharp photos of faraway objects or details of closer ones.
The focusing is fast and not as loud as I expected it to be due to the lack of USM. In fact it is easily drowned by the buzz of the average day out in the city.
So far I am properly impressed by the quality that went into this lens, and since it covers such a great range, it's definitely not coming off my 550D any time soon. You would be forced to search quite a while to find a better lens for the 18-200mm zoom range. I am recommending it without a doubt.