I own a wide variety of lenses, including the 24L II, 35L and the 24-105L. The 24-105L is a very solid lens, and I get great results from it in bright light,and particularly at f/5.6 or better.
However I have not been fully happy with the performance of the 24-105L at f/4.0. The image is not as sharp as I'd like. Also, the extra creative possibilities of f/2.8 on the 24-70 II were pretty appealing.
I went ahead and made the decision, and after two days out shooting with it have no regrets. The 24-70 II feels sharper at f/2.8 than the 24-105L does at f/4.
But what I'm most impressed about is the rendering, particularly for the OOF areas. The bokeh is loads better.... the 24-105L has quite ring-like bokeh, whereas the 24-70 II bokeh is fairly smooth across the blur disk (there is a small amount of onion-like structure if the blur area is under-exposed but not very noticeable). I now see why people compare the lens to the 24L II - the colours and rendering are fantastic.
What a great lens!