When I was purchasing my 70-200 my local camera store got in both the Sigma and the Canon so I could try them out.
In the end I purchased the Sigma 70-200 F2
.8 EX HSM and am totally happy with my choice.
The focusing is fast and silent and the build quality is excellent.
Results from the lens are stunning and it is sharp throughout the aperture range.
I also have the 1.4 EX converter and the results are great when using this combo.
The lens has just been updated with a DC version.
I cannot fault the Sigma EX range of lenses.
I have numerous uploads from both the 70-200 on its own and with the converter, have a look if you are interested to see results.