CANON 18-200MM

It really depends on what you plan on doing with your images.  If you aren't going to enlarge them much, i.e., small prints and viewing on small to medium sized monitors and little or no cropping, then you might not even notice the limitations of the 18-200.  There is distortion at both ends of the zoom range so vertical or horizontal lines near the edges will appear bent.  At 200mm it is not sharp enough to enlarge much, especially in the corners.  It is also fairly slow at 200mm so it needs a lot of light.

I started with a Sigma 18-200 OS (Canon hadn't released theirs yet) in order to have a single lens solution for a vacation to Germany.  I had some shots along the Rhine river that required cropping out at least 50% and the results were not good - definitely fuzzy.  I also had a lot of trouble with indoors shots with no flash and no tripod (not allowed).  Bracing the camera on something helped but didn't solve the problem.  Eventually I got to compare my Sigma to the Canon 18-200 and they were very close in image quality.

I ended up switching to a 17-55 for a trip to Italy where I knew I'd be shooting indoors where they don't allow flash or tripods.  This allowed me to get handheld shots at as low as 1/15s.  The images from this lens were enough better than those with the 18-200 I started looking for something to replace the 55-200 range.  I ended up getting a 70-200L f4 IS after comparing a few of the non-Ls and the f4 and f2.8 versions.  At 200mm the difference between the L and the Sigma 18-200 are easily noticeable in the center and dramatic in the corners.  Yes, the two lenses cost a lot more than the 18-200 but the results are dramatically better and this two lens combination is way more versatile.  It is a pain carrying two lenses and having to switch between them but that is the cost of wanting the best image quality.  One thing I did discover is these lenses hold their value much better than the 18-200 (especially the Ls).