When I first got the Tamron 28-75 in 2005, I took it outside the office and shot this at f2.8 to make sure it was focusing properly and to check sharpness. I had read about all the "bad" copies out there. Well, this one shot told me everything I needed to know about the copy I received:
This is what I found just a little off-center to the bottom left while I was pixel-peeping at 100%:
This lens gets little respect from hoity-toity L-pimps on all the forums. While it's construction isn't like an L, and the focusing motor is a little noisy, the thing is a marvel of an optical design, and quite an achievement at the asking price. I'd gladly buy a newer version with better build and USM/HSM equivalent focusing at twice it's asking price.
Yeah--I'm not really tempted by the 24-70L. Not until that Canon design gets updated with IS at the very least. But even then, the Tamron is the better travel lens because it is just so much more affordable, compact and non-imposing.