I compared this lens with the Sigma 18-200.
A friend of mine had the Sigma. I tested it, autofocus is very noisy as usual at Sigma, and you have the awful "compatibility errors" that strike from time to time. I have had 5 Sigma lenses in the past but will never buy a Sigma anymore. Build quality and compatibility errors have convinced me to go for another brand. Canon whenever possible. Tamron otherwise. Tokina may be right too.
I sold 4 of my Sigma's on ebay, only the 70-200 f2.8 will stay for a while.
The Tamron lens is one I use very often in travel for lightweight and range. It replaces a 1.2kg 70-200 and a 0.6Kg 28-75 2.8 and a 18-55 , hence 400gr instead of 2000gr to carry. And I never have to change the lens !
Anyway, in terms of bokeh "profondeur de champ" and sharpness, the result is not comparable to the heavier lenses. It is quite disappointing if you are a demanding person in terms of sharpness.
I was not expecting much from the AF in dark areas. Compared to Canon EF lenses with USM, this lens is much weaker, but it is ok.
However, I must admit I was a bit disappointed with the sharpness. Usually, I still carry a Tamron 28-75 2.8 XR Di for shots that require sharpness. The basic Canon 18-55 ii seems also to have much better sharpness in its range.
I think I will try a 17-85is combined with a 70-200 f4/is in the future. This combination is 500gr+700gr, which is comparable in weight, but far better in terms of sharpness and bokeh.
Another weak point of this lens : it is a terrible dust absorber. Such a big lens with external zoom can seriously dust your CMOS/CCD. I had a bad experience in India Rajastan (dusty area). The seals of the Tamron lens are probably weak. A friend with a Nikon 18-200 VR claimed he had no issue in Rajastan.
The range is considered to be +-160-170mm on the Tamron, I did not test myself. The Sigma lens is considered to have a lower range, but better sharpness.
As a conclusion, I like this lens for travel as I care a lot about weight to transport. It is a good lens, good build quality (but beware with dust !) and fantastic range. I think I will keep this lens, but will try another combi for my next travel : 17-85 +70-200 f4 and see what I like the most.