Continuing on the ever-important image quality topic ... At 18mm, CA (Chromatic Aberration) is strong mid-frame and in the corners. CA improves as the focal length is increased until basically gone at 50mm. By 135mm, CA once again begins to show mid-frame and in the corners and continues to be present through 200mm.
Watch the strong barrel distortion in the middle portion of the frame at 18mm. You will want to keep straight lines away from the borders at this focal length; otherwise, you will see a large bump out of the center with lines straightening again near the corners. To see this, look at the green trim on the corner of the building in the example below.
By 24mm, lines are nearly straight again, but pincushion distortion sets in by 28mm and gets relatively strong by 50mm through 200m. Another way to gauge the amount of distortion present at various focal lengths is to watch the top image in the ISO 12233 sample crop tool change size as different focal lengths are selected. Also, compare the 18-200 images to the nearly-distortion-free Canon EF 200mm f/2.8 L II Lens samples for the same camera.
Flare is well-controlled overall, but shows up stronger at the longer end of the focal length range (where the lens hood is less effective). Here is a Canon-Mount Super Zoom Lens Flare Comparison that shows the EF-S 18-200 to be best-in-class in this regard. Color and contrast seem fine.
Vignetting is visible in the corners at wide open apertures. It is moderately strong at the long end of the focal length range and at 18mm. Being an "EF-S" lens, the 18-200 IS does not have the larger image circle size that a full-frame compatible "EF" lens has and therefore shows more vignetting on the EF-S compatible bodies it mounts on (all have a 1.6x FOVCF).
The Canon EF-S 18-200mm f/3.5-5.6 IS Lens is relatively sharp in the center even wide open. Stopping down to f/5.6 brings nice sharpness to the center of the image through 50mm. Use f/8 from 80mm through 200mm for best center sharpness. Corners are soft wide open and need f/8 for good sharpness over most of the focal length range. Corners are decent at f/5.6 at 18mm and prefer f/11 at 135mm. Sharpness out of this lens definitely exceeded my expectations.