This is not a great lens, optically, but it is versatile. My wife likes this lens. She accepts the distortion at each end of the zoom range, in exchange for not being compelled to change lenses. My wife's employer, a medical examiner's office, issued her this this lens, along with a Nikon D300s, several years ago. When the rules were changed, to allow her to use a personally-owned Nikon camera, I bought her a D7000, and she wanted to continue using a Tamron 18-200mm Di II, so I bought this lens, too. The images shot with this lens are used for official evidentiary purposes.
As with any zoom lens that covers such an extensive focal length range, there is "barrel" distortion at the wide end, and "pincushion" distortion at the long end. It is best to avoid each end of the range, for the best images.
There is a review of this lens in the review section of the Cameralabs web site, along with reviews of the Sigma and Nikon 18-200mm lenses. These reviews convinced me to try buying a pre-owned Tamron 18-200mm to fit the Canon cameras I use at work. When I later bought my 7D, which was packaged with a 28-135mm kit lens, I stopped using the Tamron 18-200mm in low light, as the Canon 28-135mm auto-focuses more quickly in low-light conditions, and 135mm is usually sufficient at the long end of the zoom range.
Canon 7D2/7D/5D/40D/1D2N; Nikon F6/D700/FM3A/1Dx/Coolpix A; Canon 40mm 2.8 STM, 135L, 50L, 35L, 50mm 1.8 I, 100mm 2.8L Macro, 10-22mm EF-S, 400mm 5.6L; Nikkor 50mm 1.2 AI-S, 50mm 1.4G, 50mm 1.8D, 16mm 2.8D Fisheye, 180mm 2.8D, 100-300mm 5.6 AI-S, 18mm 2.8D, 45mm 2.8 AI-P, Micro 60/2.8G; Voigtlander 90mm f/3.5 SL II