The downside of any 18-200 (canon, nikon, sony, sigma, tamron, etc) is that the picture quality is lower when you compare it to two lenses (say a nice standard lens combined with a tele lens). Less detail, less contrast, smaller aperture so less light making it harder for the camera to focus and you have to use higher iso values), weaker construction (the 18-200 lenses extend a _lot_) when zooming resulting in lens-wobble and zoom-creeping.
Sure its easier, you dont have to switch lenses all the time. But dont expect the high quality pictures you see from dSLR camera's because you wont get them with such a all-in-one lens.
If you realy must use a 18-200 lens then I would go the Nikon way, that's the only pretty decent 18-200 lens out there.