I'll second the vote for the Sigma EX 70-200 f2.8. I've had mine for nearly five years, and it's one of my favorite lenses. It's really sharp, and easy to focus in dim light. The only thing I didn't like was the lens cap (difficult to remove and replace with the hood in place), so I replaced the cap with a Tamron cap. The Tamron cap has center squeeze-tabs, as well as at the edges. Much handier.
If you don't mind the size and weight, you'll really like the lens. Pat White
