I absolutely love my 28-75. It's got some vignetting on full frame, but not anything that you can't fix in raw. When i bought it I went in to
Adorama and played with both it and the Sigma. The Sigma 24-70 has a better build quality by far, but I had sharper results with the Tamron... so I bought it. I lust for the Canon 24-70 f/2.8L for my full frame and the 17-55 for my crop camera, but the money fairy hasn't left me any packages under my pillow, so... I will love my tamron and care for it carefully until she does.
My biggest complaint? It's not well sealed AT ALL. I have had to have it sent in and cleaned because of it.