I've read on other photo forums (not here), things like "my sigma 20/1.8 was perfectly sharp wide open, Bryan must have gotten a bad copy or is biased" (without providing sample images or any kind of testing setup). I definitely don't think Bryan is biased in any way, but Canon lenses definitely get the priority for review order, and there's a lot of new ones about.
There's a review of the 120-300 f/2.8 at photozone. It looks decent, nowhere near as good as a 300/2.8 prime wide open, but at 300mm f/5.6 it's near enough as good as the 70-300L.

I suppose the best thing you can say about Sigma is that their Quality Control processes need to be improved, if every lens performed the same then there'd be no 'hunting for a good copy'...