A bit late here, but I'm another owner of the 120-300 OS current version, not the S/A/C generation one, if that's even out yet?
Anyway, is it fast at AF? No. It's mediocre. Wouldn't count on BIF shots if they're heading quickly towards or away from you, but side pans wouldn't be a problem.
Is it sharp wide open? No. I couldn't quantify it, but it is not bad, but not critically sharp. It noticeably improves when you stop it down a bit. This also applies with the sigma teleconverters attached. Stop down a little from wide open and it sharpens up. More than usable wide open, but it isn't the best.
I have previously owned the EF 300/2.8 non-IS. I would rate the AF speed and sharpness as about equal. The Canon had much worse vignetting though.
Note I primarily got the Sigma to work with teleconverters as I needed to go beyond the reach of the 100-400L. In that sense it makes a great complement.
I got my Sigma about a year ago, and prices have dropped about 20% over that time especially since they announced the new version. Right now I think it is a bargain, a great lens for the money as anything else like it is significantly more expensive. The successor model is rumoured to be the same optical formula, with improvements to build quality. Take that for what its worth.
