I recently sat down with the same choice on the table for Canon side, again the 300mm f2.8 in nearly every review I read came out on top, but not by a massive difference and also its price is more than double that of the sigma 120-300mm f2.8. In the end I went with the Sigma, all I could find said that it delivered a very usable level of image quality, even when paired with a 2*TC (stopped down one stop improves things a lot).
My own thread on the topic:
http://www.thephotoforum.com/forum/...72194-sigma-120-300mm-f2-8-os-has-landed.html
Another user looking at considering the lens (nikon side as well) I've also expressed some of my early views in this thread.
http://www.thephotoforum.com/forum/...120-300mm-f2-8-ex-dg-apo-os-hsm-thoughts.html
A few threads that might be of help with your own decision
Canon 300mm II & Sigma 120-300mm OS - BirdForum
Siggy 120-300mm f/2.8 OS... - BirdForum
Sigma 120-300mm F2.8 APO EX DG OS - Canon Digital Photography Forums
Sigma unveils 120-300 f/2.8 EX OS - Canon Digital Photography Forums
Sigma 120-300mm F2.8 APO EX DG OS - Canon Digital Photography Forums
Thus far I've really not had a chance to get out with the lens properly - though I've taken some really dull building shots and can say that I'm very pleased with the results. I'll try and get some more today (maybe a bird or two if the crows are in an agreeable mood) to post up.
One thing I will say is that the lens is most certainly a heavy beast, I suspect one can grow used to the weight and with repeated use use it longer handheld without problems, but I think for an all day shoot with this lens you'd be greatly helping yourself with a solid monopod to take the majority of the weight (the sooner fatigue kicks in the sooner it harms your results, so anything to hold it off helps).
The OS keeps up well, though might be a touch pushed when matched with the 2*TC, but I say that having only shot 600mm very recently (up till now my longest was 400mm) so some adjustments to shutter speeds, limits and holding posture should improve things.