Summary: I was using a kit zoom and I was out in on a windy day, when I got home I noticed dust in the View finder. I took the lens off; I saw dust throughout my camera even on the sensor. I had to buy a new focus screen; the old one couldn’t be cleaned.
Conclusion: Since I traded the 24-105L for this lens, it's by far my most used lens. Sharp, fast, flexible, 24mm is great on FF. It's big and heavy, but that doesn't bother me at all (I shoot with a 1DM3 + 70-200 f2.8L IS for 16 hour days).
Summary: I bought this lens as my roundabout lens for a 60D and sweated at paying over $1300 with filter after reading about the bad copy syndrome. Don't BUY into that. The lens came through as described with pinpoint sharpness.
Conclusion: I originally had the Sigma EX 24-70mm f/2.8 lens and found the image quality great for the price paid, however it did struggle with autofocus. I paid 3x as much for the Canon 24-70mm f/2.8L, and yes the autofocus was much faster, and while the image quality wasn't that much better than the Sigmas,...
Pros: Contrast, flare, accurate AF, sharp at f/2.8, solid build