Excerpt: Bought the Sigma F/3.5-6.3 18-200mm Lens as I has a mishap with my Sony 18-200mm. The price was very good and my experience with the 18-200mm as a "walk around" lens has been great. I must say the Sigma lens was far better than I expected.
Pros: Consistent Output, Durable, Fast / accurate auto-focus, Lightweight, Nice Bokeh, Sharp Photos Throughout
Summary: I bought this for my Pentax K-30, and one of the factors leaning me towards this lens was the OS or optical stabilizer. When I received the lens it turned out that the manual says "The optical stabilizer is a feature not supported for Pentax cameras" and the title says OS and PENTAX.
Summary: Happy with purchase but yet to realise its full potential - still experimenting. Have the Nikon 18-300mm but this is a bit lighter and easier to carry around which was the reason I bought it actually
Summary: This lens is heavy and doesn't seem to focus as fast and cleanly as my Canon lenses. This is annoying because I mostly use it for low light action shots with occasional object between myself and the subject. It works, but get the brand name if you can afford it.
Summary: I love having an all-in-one lens for photographing children and families indoors and out! It focuses great ad long ad there is plenty of light. In low light, I get more out of focus photos, but no more than I used to with my kit lens.
Summary: This is an easy to use lens, and it takes sharp pictures, even those at a distance. It does however, vignette the pictures (shadows at the corners. But since I often recrop my pictures, this isn't a problem.
Summary: The lens is actually the perfect walk-around lens as it covers a very wide-range in terms of focal length. However, since it's a third-party Sigma lens, it's glass is not the best, if not the worst. You can't really take artistic pictures with this lens as its colors aren't very nice.