I really think Canon should have improved the 24-105 f4 L and left this one on the shelf. The only appeal of the range is if you have a 2.8. At this price I would rather invest in the Tamron 24-70 f2.8.
Yeah, and with the Sigma 24-105 out now receiving excellent reviews I see the place for this lens a little less. *except* for how compact it is.
So far the Sigma has had one great review.... from the same place that says the 70-200 2.8 II delivers the worst 200mm f/2.8 performance of all the Canon 70-200 lenses.... and the 70-300 non-L beats the 70-300L and 300 f/4L.... and the 16-35 II has the sharpest FF edges at f/2.8....
Don't forget how they rank lenses either, they take one aperture and focal length, whatever performs thebest (which strangely seems to be wide open 99% of the time) and compare. So you could have a sigma doing well middle range wide open but worse in other regards (like say at 24mm where it's tricky to do well on FF) getting ranked higher, for instance.
I mean maybe the sigma will prove to be good, but I'm still wary of it based upon their own MTF charts and early sample photos.
It is also larger and heavier than the 24-70 II 2.8! despite only offering f/4.