I'm happy with my Sigma 70-200 f/2.8 EX HSM. It focuses fast and is sharp. I have no doubts that the Canon counterpart is a little sharper at f/2.8. I bought my lens used from B&H for $550. You can see some images I shot with this lens at
http://www.phototalk.net/rryerson - The images are in the Motorsports folder. I have other images, such as portraits, that I have shot with this lens but I don't have them up on the site yet.
-Richie