I have said early on that if your shooting style
requires carrying just one lens (e.g., vacation where carrying and changing lenses is difficult), lenses such as 18-200mm makes sense. These lenses exist to fulfill such needs. Otherwise, why would anyone buy anything but super zooms? And yes, I admit that Nikon's 18-200mm is one of the best performing lenses in super zooms category.
If taking fantastic looking photos is a priority, such as razor sharp image quality, minimal distortions and image defects, fast maximum aperture for low light shooting, creamiest of bokeh, etc., then you can do much better with more specialized lens.
Obviously, building a collection of fixed focal length will yield the best results, and many photographers use only these lenses. But most of us prefer, especially those that earn cabbage from something other than photography, versatility over image quality.
On Canon, Sigma AF 18-200mm f/3.5-6.3 DC OS comes closest to matching Nikon. That said, maximum aperture at telephoto is less (f/6.3 vs. f/5.6), bokeh is generally reported as poor, build and image quality are worse, and image stabilizer is not quite as effective as Nikon's. If you don't need wide angle photography, Canon's EF 28-135mm f/3.5-5.6 IS USM is an oldie but still a goodie. Conversely, if you don't need telephoto, Canon's EF-S 17-85mm f/4-5.6 IS USM is a popular choice.
In the end, you will do better carrying two lenses, such as:
- Canon's EF-S 18-55mm f/3.5-5.6 IS (new kit lens for XSi) and EF-S 55-250mm f/4-5.6 IS or Canon EF 28-135mm f/3.5-5.6 IS USM
- Sigma's AF 18-50mm f/2.8 EX DC Aspherical IF and AF 80-400mm f/4.5-5.6 EX APO OS
- Canon's EF-S 17-85mm f/4-5.6 IS USM and EF 70-300mm f/4-5.6 IS USM
- Canon's EF-S 10-22mm f/3.5-4.5 USM and EF 24-105mm f/4L IS USM
- Canon's EF 17-40mm f/4L USM or EF-S 17-55mm f/2.8 USM IS and EF 70-200mm f/4L IS USM or EF 70-200mm f/2.8L IS USM