generally, you get what you pay for. You have several categories that define a lens. Focal range, aperture, build quality/optics and price. Price is a result of the combination of the frist 3. the 18-200 range at the pro level is generally composed of 2 to 3 lenses. someting like 12-24, 24-70 and 70-200. Each of which is over $1000 with aperatures of 2.8 or so. To keep the price down and include all those focal ranges, the sacrifice went to build quality and aperature (as a comparison, each of those lenses is probably bigger and heavier then the 18-200). Not saying its a bad lens, just may not be adequate for what you want to capture. Depending on your intented use, it may be slow or soft. On the other hand, it has a very comprehensive focal range. I think the 18-105 is a great focal range. If you were to buy a second lens I would go for the 70-200 f/4. Same price as the 18-200 but is better at the dedicated ranges as it aims for a zoom telephoto lens rather then wide to telephoto zoom. The benifit of a large focal range is counter balanced by the benefit of the more specialized lenses as far as wide aperatures, image quality and sharpness. Hope this helps your decision/info gathering process,
Nick