I have both, may sell the 24-105L. I suspect that a 'walkaround' shot from the 24-70 II at 70mm cropped would equal or beat the IQ of the 24-105 at 105mm (which is where the lens is weakest).
Actually, the utility I can see for the 24-105 is for portraits in a studio-type setting. While a 70mm image can be cropped, the perspective is not going to be the same as a shot at 105mm. The shot at 105mm will be a much more appropriate perspective for a tight portrait. In a studio setting, there's plenty of light and the background is controlled - so f/4 is fine.
Interesting, Ive always heard it sucked at 24.
On the 60D it is great, no issues at all.
But on the 5D, there is definitely some distortion and softness toward the edges.
Still, despite those flaws, I find it a much more versatile lens on the 5D than the 60D. Although the lens gave more reach at the telephoto end with the 60D, it often wasn't enough for true telephoto shooting so it could be a bit awkward sometimes...