...if you're going to seriously complain about a lens of this high a quality as it is, then just Join those who have jumped on the Leica train. Oh, and be sure not to buy any inferior Voightlander products.. my god the colour rendition! the horror! it doesn't say Leica Summilux! Here's a tip for everyone. If you're shooting an APS-C sized sensor, don't complain about the quality of the lens unless it's REALLY that bad.
Anyway, the optical difference between the Tamron 17-50 and something 'higher quality' like the Canon and Nikon 17-55 is soo minuscule in each of these areas...
In fact, at the wide end, the Tamron is less distorted than the Canon... and the lens only offers maybe a SLIIIIGHT bit more vingetting than the Canon... and it gives the same speed in a more compact, lighter package.
really, there's no reason not to get it if you have at least 300$ to spend on an APS-c standard fast zoom and you're not a snob against 3rd party glass.