I pre-ordered Canon's new 24-70 zoom when I bought my 5D Mark III. After many delays, it finally arrived yesterday.
At lunch time today I went to a spot that has a decent view of central Sydney and shot a series of photos to compare it to the following lenses at F2.8, F4, F5.6, and F8:
24-70 F2.8 L (at 24mm, 35mm, 50mm, and 70mm)
24mm F1.4 L II
50mm F1.4
I used a tripod and mirror lockup, checked the focus with live view, and shot at least two series of photos at each focal length, refocussing between each series.
The results were entirely satisfactory. The new zoom was clearly superior to its predecessor at every focal length and aperture. It was even significantly sharper than the 50mm F1.4 at F2.8, although the prime lens perhaps had a small advantage at F8. It was slightly inferior to my 24mm F1.4 II, but the difference diminished as the lenses were stopped down.
I was never 100% happy with the original 24-70, and prefered to use my 24mm and 50mm primes for superior image quality. With the new 24-70 there's probably no point carrying the 50mm F1.4. (I very seldom open it wider than F2.8) The 24mm F1.4 II is still worth having for the extra speed, but unless I expect to shoot at F2.8 or faster I'll leave it at home.
It's nice to see that Canon's optics are improving at a decent rate, even if their sensor technology is falling behind.