I've tested this lens against a 24-70L, 35L, 50/1.8 and a 70-200/2.8L.
Tamron beats the 24-70L at 28mm, 35mm, 50mm and 70mm in the center. In the corners, they are neck-to-neck. The Tamron is more contrasty but also shows more CA.
Against the 35L, the Tamron loses in both in the center and corners, but only by a hair.
The 50/1.8 beats the Tamron in both the center and corners. Ditto for the 70-200/2.8L.
The surprise here is the poor performance of the 35L, but maybe I have a bad copy. Anyway, for around $400 (got mine at a local store) the Tamron does not disappoint at all. It may not have the solid build of the 24-70L, but it is more than good enough for everyday use. I've noticed that the AF hunts more than the L lenses, but this shouldn't be a problem if there's more contrastly areas nearby. The same could be said about the CA issue, which can be easily corrected in PP.
So, all in all, if you're looking for a fast yet light walk-around zoom that doesn't give up image quality to lenses 3x its price, you can't go wrong with the Tamron 28-75.