I wonder if this explains the long, unexpected production delay and finally they just gave up and started shipping.
This is my thinking as well.
I looked at a few (ended up with more than one to insure I had a copy for a certain shoot), each copy was different. On the plus side, all were better than all three 24-105 I've seen. On the minus none performed the same as any other copy, all placed DOF of the edges and corners, espc. at wider side differently and all had different 70mm wide open center frame performance. Even the worst at any aspect was still good, but for $2300, yeah it would be nice to have everything 100% the best and not this is better but that is worse and that is a little worse and then that is better.
The one I decided to keep in the end is amazing 70mm wide open center frame and pretty good at wide edges and corners (I think, it's hard to tell which is the way the DOF should be placed) although lower left corner might be a touch soft. Maybe 3 more copies and I'd get something perfect in every last regard?? As it is though, it's good enough to dump my 24 1.4 II over and it has the sharpest 70mm f/2.8 center frame I've seen from any lens, so not so bad (70mm far edges are weaker than my 70-300L on this and all the copies looked at though).
I think a lot of people are hoping that this can replace a few primes. How many copies did you go through total?
I saw three. Two were VERY sharp 70mm 2.8 (one just beyond belief sharp there and even better than the other, an absolute stand out) and one a bit less so, difference can be seen real world (especially comparing best one to the worst one there), the worst was still good though, better than a good tamron 28-75 and at least as good as a 70-200 f/4 IS wide open 70mm (granted that is the weakest spot of the 70-200 f/4 IS). With two of them it was easier to get to deliver sharp 24mm edges in real world scenes, it seemed, and one had seemingly placement of the DOF at 24mm (the one that was inbetween at 70mm f/2.
that made it a bit trickier (although focused on a chart, re-focused at each spot did well there; all three just placed the DOF around the main center object at different depths at the edges compared to each of the other copies. I probably should have set up a test with boxes at various distance left and right and then focused on something in the center and seen how DOF fell on each side with each since with my complex scenes it was hard to say for sure which copies were placing it in the most expected fashion, it did seem that two of them were a bit easier to get real world shots working as wished).
So it's a little hard to say which were better, for sure, with what I did so far, but I can 100% for sure they are different, different enough to repeatably spot, which probably shouldn't occur at $2300 for a standard zoom. But then again, overall, all still seemed better than all the 24-105 I've seen, certainly at 24mm, all the differences notwithstanding. So the copy variation does seem kinda high for such a lens and yet they all still deliver as well or better in all regards examined so far (and I have not compared much center range at all so far) than any other standard zoom anyway it seems so it's maybe not really as bad as it sounds though in the end when it really comes down to it. All copies could do some pretty amazing things for a zoom.
I didn't look at 35mm edges, looking at TDP, maybe that is something that should also be tested with these.