Anyone heard anything good about:

Tamron AF 18-250mm f/3.5-6.3 XR Di II lens

Sigma 18-200mm F3.5-6.3 DC OS

vs Canon and Nikon 18-200mm lenses?

I heard that the standard twin lens kits that are selling cheap these days are not that good. The lenses above are cheaper or as cheap as the Canon and Nikon equivalent 18-200 lenses.

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