Summary: Comparing this to the Canon equivalent, it was much cheaper, with better quality optics, good build quality and fast focusing. The extra 1/3rd of a stop on the telephoto end isn't too much of a big deal either.
Summary: Purchased used through Amazon. Nice lens, good price used. Built well. Little fuzziness at full zoom, but very sharp in the middle of the zoom range with a small aperture. The autofocus can hunt and is a tad slow, but no worse than the kit lenses that come with the body. Glad I purchased it.
Summary: I recently upgraded my camera from the T3i to the 70D. I had the Canon 18-200mm lens, but wasn't always thrilled with it. It was heavier than I wanted, and it had a problem with lens creep that would drive me nuts. I sold it on ebay. Got my Sigma 18-200 a few days ago and I'm already in love with it. All the pictures are crisp, even the low-light pics I'm testing are very reasonable.
Summary: Actually is not a bad lens for the price. I consider it my all around lens for traveling (one lens vs one wide, one mid and one tele). Excellent performance in the day light but not so good on low light. This lens is little bit slow. I was expecting more out of this lens but overall I consider it a good lens once you learned the lens limitations. Happy photo shooting!
Excerpt: Very practical for travel, children and, in my case (I have two wide angles), using it as telephoto.<br /><br />The lens is very well built, but the focus ring is a little loose. Sharpness is pretty good if you stop down, but wide open could be better.<br /><br />See below two images (@100%): i) center (@200mm) and ii) extreme corner (@200mm).
Pros: Sharp From F8 On, Useful Macro Capability, Useful Range 18-200mm, Well built
Summary: When it comes to the perfect "walk-around" lens this one fits the bill. There are now 3 versions of this lens (a new one coming out every couple of years) but this one is by far the best. I was very impressed with the last model but this one performs better in all areas. I've had the Canon 18-135, 18-200, and the 28-135, the Canon 28-135 was probably the best competitor to this lens however the 28mm on a crop sensor was too cropped for me.
Summary: As a landscape photographer, sharpness is irreplaceable. I am always on the look-out for a light lens that suits my backpacking and hiking needs - and this lens is not it! Realistically, I knew this lens would not perform like a prime lens or $1500 zoom lens. However, it is incredibly soft and that is a deal breaker for me. In comparing with my Nikon 16-85 and 70-300 on my Nikon D7100, images were notably softer across all focal lengths.