I have to say, my first thought when I saw this was the same as that of many others, judging from the comments here. I just traded my 70-200 f/4 L IS in for the f/2.8 version, BUT not because of poor performance.
The lens already has stellar image quality (sharpness (Oh yes!), and bokeh) and IS.
The 24-70 L, although I've never even used one, from what I've read about it, could really use the attention, and to be honest, although I do love my 24-105L IS as a walk-around, it does suffer from CA at the edges, and it could use an upgrade to fix the edge/corner performance and sharpness generally.