I have this Sigma one day compared directly with the Tamron 18-270mm F / 3.5-6.3 Di II VC PZD (see my review here). In terms of image quality, the Sigma has clearly ahead. More sharpness and contrast than the equally priced Tamron convinced me. Also, the image stabilizer is the Sigma wonderfully quiet and yet very effective (2-3 stops). Very nice is also the small size and low weight, when compared with the significantly grüößeren and 200g heavier predecessor. Only the somewhat short path of the focus ring for manual focus makes it difficult to focus manually something and therefore costs one point deduction. Here Sigma could spendieren something more "Fokusweg".
Otherwise I can only recommend the Sigma. In particular, compared with the Tamron 18-270mm F / 3.5-6.3 Di II VC PZD is beyond me, who would prefer the Tamron at the same price the Sigma.