Have a look at http://pbase.com/mortain/sigma_18125_examples
It might help. I use it all the time. The stiffness disppears, and I just don't notice the noise. I would like a sharper lens but I do not want to change lenses for different shots, so there is no alternative. Minolta are bringing out an 18 to 70 later this year, and as Minolta lenses are actually rebadged Tamron, I await the Tamron 18 to 70 with interest. If it is as good as the 28-75, thenthat is the one for me.
ken from the uk