I was wondering if I should sell my old 24-70mm 2.8L and get this new lens. Has anyone make a side-by-side comparison in terms of IQ?
All of it is here (at least from a sharpness perspective):http://www.lensrentals.com/blog/2013/01/canon-24-70-f4-is-resolution-tests
As you'll see the F/4 IS is better than the 24-105 and 24-70 I, but not as good as the 24-70 II.
I made the jump from the F/2.8L I to the F/4 IS and I'm quite pleased. I don't shoot indoor sports, and I don't shoot portraits with that lens, so I had little need for F/2.8 in a standard zoom.
The new F/4 in comparison to the F/2.8L IS is sharper, lighter, shorter in length, and has a very serviceable macro mode if you are in a pinch. It's a perfect all-in-one lens for travel, hiking, etc. I am very pleased with the pickup. And on a lesser note, it doesn't push me to need to get 82mm filters. Unless I'm shooting with my 100L macro, all my lenses are 58mm or 77mm, which is a nice convenience.
Be advised that the lens does take a toughness downgrade
compared to the venerable 24-70 F/2.8L I. The Mark I is a tank with a lot more metal. The 24-70 F/4L IS is not poorly constructed by any means, but it's not built for war. I'd liken it to the 24-105 or the 100L macro -- it's plasticky, but solidly assembled.