Roger and Aaron at LensRentals.com have completed a comparison test between the Tamrom 15-30 VC f/2.8, Canon 16-35 f/2.8 II and Nikon 14-24 f/2.8. Some of you may find the results quite surprising. One thing is for certain, the upcoming Canon EF 11-24mm f/4L better be extremely impressive if it costs almost 3 times more than the Tamron 15-30mm f/2.8 VC while being a stop slower.
At the wide end of the zoom range, the optical differences are complex. In the middle 2/3 of the field the Tamron holds its own or is a bit better, but it's not quite as good in the corners. At the long end (30-35mm) the Canon lens is better than the Tamron. At 23mm, though, the Tamron is as good as the legendary Nikon 14-24, and clearly better than the Canon 16-35. And, of course, the Tamron comes at a significantly lower price than either of the manufacturer's lenses.