I'm going to recommend against getting this lens. I have a personal preference against lenses that cover a very large range of focal lengths, but for a reason.
It's impossible for lenses to be optimized for very wide (18mm) while still perform well at very telephoto (200mm). In this case, I would expect the 15-200 lens to perform well between something like 20mm and 160mm. At the ends of the focal lengths, contrast and sharpness will likely be pretty low.
If you know how to read MTF charts, check out:
I am bad at explaining them...
I have the 70-300mm Sigma and it has a similar MTF characteristic. The ED glass helps a lot, but just don't expect a lot when you're shooting at either end of the lens. That comes with the price I suppose! =P
I'd recommend something with a bit smaller range. What lenses do you already own?