I have used both the Sigma 70-200 OS HSM and the 70-200 Mk2.
In my personal opinion - the Canon is sharper at all relevant apertures than Sigma, but this is MOST noticeable only in the corners. At the centre it is VERY very close, after f3.2. At 2.8 it is noticeable but only if you compare side by side. For a lens that cost 45% of the Canon(here in Singapore), the Sigma performs VERY well.
I’ll like to point out also that the differences are really pretty minor for the part of the frame you likely will use, so if price is a big problem for you then the Sigma will save you a pretty penny.
Some things to note about Sigma’s focusing implementation.
1. Speed is almost the same as canon, with a slight edge to canon if I have to pick a winner.
2. Both are equally silent
3. Sigma does not have a focus limiter, so in lower light, at long range it will seem to hunt longer than the canon as it will hunt the full range, rather than the canon which will only hunt across the limited range. This to me is the main reason why many people feel the Canon focuses faster. If you turn off the focus limiter on the canon, both will hunt for pretty much the same amount!
4. Sigma’s Stabiliser Mode 2 for panning doesn’t detect orientation, hence panning mode only works in landscape mode, while the Canon works in both landscape and portrait panning.
If you can live with these caveats, I guarantee you will LOVE this lens like I did. I eventually kept the sigma and used the extra pennies to grab a Sigma 85 1.4 which is SUPERB and without peer for its combination of IQ and speed. That prime is now my favourite lens!