I am getting very favourable reports from photographers who are using this lens.
I can only assume that Canon have intended
this lens to offer better 'IQ' than the 24-105, though not all early testing has confirmed this. It's interesting to note that here in the UK the RRP of each lens is only £20 different, but the actual selling price of the 24-105 has fallen to the £850 ( about $1,300 ) region, still much more expensive than in the States, whereas the 24-70 f4 has only dropped to £1299. ( Bearing in mind that gear such as this is proportionally much more expensive in Europe than America ).
We may be purchasing one for Building Panoramics, so if we do I will post images and further thoughts on the lens. ( We are aware of the RSA issue).
As has been pointed out many times, there's more to image quality than test charts