This shouldn't be a surprise, but to read all the in-depth reviews of the Sigma 24-105 Art and the Tamron 24-70 F2.8 you could be forgiven for thinking that either of them could give the Nikkor Trinity 24-70 F2.8 a run for its money.
Loath to repurchase a Nikkor that I sold only a few months ago, I tried one copy of the Sigma and two of the Tamron on my new D810. The Sigma wasn't terrible but it was skewed and had pretty poor corners. Both of the Tamrons were very skewed, one almost unbelievably so. SO I bit the bullet, took it like a man and upped the ante for a Nikkor.
It is GREAT. Better than the one I had before, in that by F8 the edges are acceptable if not brilliant at 24mm and at 70 it is really quite acceptable. But at 28, 35 and 50, especially at 50, it is really really nice. Worth the money, easily.
I wonder if, when all the review sites get sent copies by Tamron and Sigma and so on, they get cherry picked good copies. It seems likely to me because they do review really well. But factory production copies are in my recent experience really are pot luck and in the end not worth the saving.
The Nkkor is a fine companion for the D810 and if anyone is looking for a mid-range zoom, given my previous experiences with the 24-120 too, it is the best game in town.
And thanks to the ever brilliant Park Cameras dealer for never questioning my judgement on this and for handing over replacements with good grace and humour...