The Canon 24-70 f4 L IS seems to be treated like the red headed stepchild on the internet. It's not easy to be the little brother to the Canon 24-70 F2.8 II lens. I first borrowed 24-70 F4 lens for a 2 week trip to Glacier National Park. The lens was superb on my 5D3. Easy to hike with and IS. I liked it so much that I bought one when I got home. On the 2 copies that I have used of this lens, both have been very sharp, fast accurate AF and great IS.
I have used the Canon 24-70 F2.8 II and IMO the 24-70 F4 gives up little in IQ to his bigger brother.
I have found in my use of this lens that the 50mm focal point is quite contrary to popular opinion on the interenet, the 2 copies of this lens that I have used were/are very sharp.
This Canon 24-70 F4 L IS plus my Canon 70-200 F2.8 II make for a superb combination. Same filter size. Both have Canon's newest IS. Superb image quality and sharpness.
If you don't really need the f2.8 aperature and want IS, this is a lens you really should try.
The much loved Canon 24-105 of course is another option to consider, especially given the price of it now. I chose the Canon 24-70 F4 over the 24-105 for it's much better handling of distortion at 24mm and upgraded IS.
I dont miss the extra reach of the 24-105 because I carry the 70-200 for that. That's my real world review of this lens.