I just picked up this lens after reading the positive reviews in this forum. I love it. I also considered the sigma 24 - 40 2.8, but my local camera store didn't carry it and my initial impressions of the Tamron at the store were very good. My copy is incredibly sharp, even at 2.8, coming close to the performance of my 50 1.8. The build quality seems pretty good, certainly a step up over the plastic 50, but still not nearly to the level its Canon L counterpart. Focus speed also leaves something to desire compared to Ring USM.
I happily gave up some build quality and focus speed over the 24-70 L in exchange for a lens that, at least in the case of my copy, is optically almost equal and weighs almost a pound less. The $800 I have left in my wallet also probably has something to do with my happiness.