Sigma 70-200mm F2.8 EX DG Macro HSM II Lens Review: 1. Introduction: Digital Photography Review
The lens premiered in 1999. Re-designs were done in 2005, 2006, and 2007. Read the conclusion here. SOme optical weakneses at the long end, and suffers from "relatively high levels of chromatic aberration." Note that they say the Tamron 70-200/2.8 has "superior optics", but that the Sigma has the superior autofocusing system.
Canon 70-200 f/2.8 L IS-USM reviewed here
Canon EF 70-200mm F2.8 L IS USM Lens Review: 1. Introduction: Digital Photography Review
They did head-to-head tests of this Canon lens with the first-generation 70-200 f/2.8 VR Nikkor; see here for the conclusions on the Canon;s image quality and performance
Canon EF 70-200mm F2.8 L IS USM Lens Review: 6. Conclusion & samples: Digital Photography Review
the Canon 70-200 f/2.8 L- IS-USM is the better lens on full-frame, the Nikkor is the better lens on APS-C. I wonder if perhaps the 70-200 f/4 L-IS USM is not optimized for crop-body use, since it is the absolute newest of the several 70-200mm zooms Canon currently has. The 70-200 f/4 L-IS USM is a very,very strong performer,and I wonder if it might actually be optimized for APS-C bodies,which demand a higher-resolving lens than FF bodies require; Canon going to ultra-high MP counts APS-C bodies like 18MP with the 7D make sme wonder if perhaps their 70-200 f/4 wasn't designed with the smaller, high-density, 4-5 micro pixel pitch sensors in mind...
If you own no 70-200/2.8, the step up in performance to any maker's 70-200 2.8 is going to be a really big step up. There's always a price to be payed for the last 15-20 percent of optical and mechanical performance delivered by the $1,800 manufacturer's lens, as opposed to the 3rd party lenses that deliver 80-85-or maybe even 90 percent of the performance, but at a thousand dollars less.
I own the Canon 70-200 2.8 L-IS USM and the Nikkor 70-200 VR: I do not own the Sigma. I have shot with the 70-200 f/4 L, the pre-IS version,and it's 67mm filter size and slender barrel make it very nice and handy.