While I only own this lens a few days, it has already become my favorite. Well, my 70-200IS is still on top, but eh, I just like the size of it. 8)
Back to the 24-70. I was VERY skeptical about buying this lens due to the number of bad reports I have read. Well, I really needed the 2.8 in this zoom range, and decided to bite the bullet after contemplating it along side the Sigma 24-70 for months. All of my fears are now gone. This is simply an amazing lens. Maybe I got a "good" copy, but this lens is fast to focus, spot on even wide open, and produces some of the best out of camera pictures I have ever seen. Size and weight? Well, let's just say I don't mind walking around an event all day with the 70-200IS, so this isn't an issue at all for me. Frankly, it feels perfect on my 10D. Balances nicely, and makes the camera feel really good in my hands.
My personal final words? If your in the market for a zoom in this focal length, and don't mind putting out a bit of a hefty sum, you WON'T be sorry. This is simply one fantastic lens!