Upload & Sell: Off
Well, you've already made your purchase, but in case anyone else is looking I will add to the feedback above.
The original 150-500 Sigmas had some issues. There was a version change and new ones (as of Feb 2011 IIRC) are supposed to be 'fixed'. I was all but ready to pull the trigger on one of these when the Canon Refurb store got some 100-400s in stock during a 15% off, free shipping sale. I did play with the Sigma and see some photos one of the guys at the camera shop took. It looked very good, even at the long end. After taxes, the 100-400 on sale was only about $200 more than the sigma, which I thought was too good to pass up.