This lens is regularly scorned for being soft wide open, being too heavy, and not having IS. With my 5D Mark I, I usually used my 24-105mm f4. It's lighter, has more reach, has IS, and was just as sharp as the 24-70mm wide open.
That was until I got a 5D Mark II. I discovered that my 24-105mm wasn't as sharp as I thought it was. Wide open it was soft and gave me the same detail as I was getting from my 5D Mark I. I put the 24-70mm on it and even at f2.8 it was sharper and delivering more detail than the 24-105mm was at f4. Since the main point of getting the Mark II was the improved resolution, that made me switch lenses.
I'm sure plenty of people have gotten soft copies of this lens since Canon doesn't do QC any more but I bought mine locally and did my own QC before declaring it good.