I've got the 24-70 F/4 a couple of years ago to use on my 1Ds II and I love it. (also have the 17-40, 70-200 F/4 IS, 50 F/1.8 and 85 F/1.8)
It's sharp across the range, starting from F/4. I'm not a pixel peeper, so it can be that 50mm is a bit less sharp as some test say.
It does have a bit distortion as Canon EF 24-70mm f/4 USM L IS - Full Format Review / Test Report - Analysis says, but for critical photo's, the distortion can easily be corrected using Canon DPO. (Just like any modern mirrorless does)
Never used the F/2.8, but what I love in the F/4 compared to the F/2.8 is the image stabilizer and the fact that it's a quite usable macro lens. (and of course it's cheaper and lighter)