I did a quick comparison of the Tamron 24-70mm f/2.8 VC and Canon 24-105mm f/4L IS at 24-70mm on my 5D Mk III to determine how sharp each is in the center and in one corner. Check out the results here:http://www.larsparonen.com/2012/05/10/tamron-sp-24-70mm-f2-8-di-vc-usd-vs-canon-ef-24-105mm-f4l-is-usm/
I've attached a sample.
I also compared the Tamron at f/2.8 to f/4 at 24mm.
The short of it is that I'm pleased by how the Tamron performs. Although I've read reports of variations, the good copy/bad copy ratio has yet to be credibly established. And Tamron may have a winner here.
Apart from the results, the test seemed to confirm other reports that one or both of the lenses don't conform to the focal lengths stated across the 24-70mm range. The camera remained stationary during the test, mounted on a tripod.
Also, the Tamron let in more light at f4 at all the focal lengths tested.