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I would refer you to the review of the lens at Photozone.com.
An excerpt from the conclusion of the review states: "The Canon EF-S 18-135mm f/3.5-5.6 STM IS is a pretty obvious choice for those users seeking for a long universal range while keeping a very decent image quality. The new optical design with a large diameter UD element in the front lens group obviously makes a difference compared to its predecessor. The lens is generally capable of keeping a very high center to corner resolution. There's only a slight drop in quality at 135mm at max. aperture."