I've been wanting a faster standard zoom lens for some time now. The 24-105 4.0 Cannon lens that came with the 5D Mark iii has been great, but I was always just a bit disappointed by its speed. My goal had always been to get the Canon 24-70 2.8 L Series v. 2 lens, but that beast of a lens is around $2000 - quite a reach! I began reading about Tamron's 2.8 24-70 and found the reviews on it to be very good.
Compared to the Canon version, the Tamron is built a little less solidly and isn't quite as sharp... but the price of the Tamron isn't just a little less than that of the Canon... IT'S A LOT LESS!!! This Tamron lens can be found quite easily for $800-$900 online - more than half as much as the Canon. And no, this lens is not half as good as the Canon. In fact, it's almost as good... just not quite. The tiny difference in build and performance doesn't justify the ridiculous difference in price. In fact, I would say pound for pound the Tamron owns the Canon when it comes to value per dollar.
And if that wasn't enough, the Tamron actually does have a step up on its first party counterpart. This lens comes with vibration control so that the images it takes are stabilized. I am so used to Canon's Image Stabilization on their lenses that I find it to be an extremely desirable feature. The very expensive Canon 24-70 2.8 lacks image stabilization.
I've only had the Tamron 24-70 2.8 for a couple days, but I am already very happy with it. I have shot with it and find that it feels just as a professional lens should feel. I am very happy to add this lens to my arsenal of lenses and am now heavily considering a super-wide angle lens from Tamron in my near future.