That "cheap" Sigma 18-50 is sharper than a Nikkor $1500 17-55... has lower distortion, less CA, less pin cushioning and a free macro mode tossed in for the same price (no its not a 1:1 macro but for the price, but come on... neither the Nikkor nor Tamron even have a macro mode nor focus on an object that is a 1/2 inch from the front element so there! LOL! ).
The Tamron cannot say that. The Tamron also has one issue that irks me... the screws on the inside element come loose over time and often even drop out. This doesn't happen on all of them, but about 25% of the Tamron 17-50 Flickr users who bought one about 1-2 years ago (when I was doing my homework on which to get), reported this issue. It was relatively easy to fix, just a watch maker's screwdriver in a comletly dust free environment and a little automotive locktite along with 20-30 minutes work solved it... but I do not understand why I as a new lens owner would need to do this... becuase this was not covered under warranty and anyone that sent it back to Sigma received a $125 invoice.