Overall, we're extremely impressed with the Tamron 24-70mm F/2.8 VC. It demonstrates that it's entirely possible to add an effective image stabilization system to a lens of this type without a significant drop in quality elsewhere. The optics are excellent, the autofocus fast, silent and accurate, and the build quality very good indeed.
Overall the Tamron comes so close to the much more expensive non-stabilized Canon and Nikon lenses, both optically and operationally, that it's difficult to see why most enthusiast photographers might choose to buy them instead. That's not to say that there aren't any differences - the Tamron's not quite so quick at focusing, and chances are it's not as well weathersealed - but quite simply it offers much better value for your money.