Try before you buy
Go to a reputable camera store. They should let you stick an 18-200IS on your camera and fire off a few frames at different focal lengths and apertures. Take a look at the images and compare with what you get with your current lenses. If you're in a hurry you can even bring a laptop with you and make up your mind in the store. Your opinion of the image quality matters a lot more than internet reviews or a lens' reputation.