DPReview posted their review of the Tamron 24-70 VC, giving it their prestigious Gold award: http://www.dpreview.com/lensreviews/tamron-24-70mm-2p8-vc-usd/
DPR's evaluation agrees with DXO's, which found that the Tamron was "impressive," and even outresolved the vaunted Nikon 24-70 2.8G (15 vs. 14 perceptual mp). http://www.dxomark.com/index.php/Publications/DxOMark-Reviews/Tamron-SP-24-70mm-f-2.8-Di-VC-USD-review-A-uniquely-versatile-pro-worthy-alternative/Comparisons
(In fact, in DXO's comparison, the Tamron bested the Nikon even though the Tamron was on the Canon 5D2 which has 21mp while the Nikon 24-70 was on a D3X which has 24mp.)
The most significant drawback noted by DPR is vignetting, which for some purposes like portraits, is not a big deal.