I was looking at Ken Rockwell's site again today, and ran across his high-ISO shootout among many different cameras. The article is focused on signal-to-noise ratio as available light goes down, but what caught my attention immediately was him saying that the D4 sucks rocks. Now, I haven't pushed mine, in terms of ISO (I limit it to 6400), but I have been uniformly pleased with the results.
So that made me a bit more attentive to what he had. And what jumped out at me was the amazing lack of detail on his D4 pictures, all the way down to his ISO 200 shot. And that definitely doesn't match my experience, leading me to wonder if his is defective (seriously; his shots have less detail than the D90 that my D4 replaced would give).
In any event, I decided to put together my own ISO comparison (D4-only; I don't have a slew of cameras to which to compare. My wife had enough of a cow about the D4, which is only my third camera. My first does not deserve to be talked about in the same breath as the other two). My personal reaction is that those up to 25600 are probably usable (and, now that I've compared, I think cutting off at 6400 was a good choice), with the higher ones being usable only at web resolutions (and, for the highest, only very small web resolutions). Yes, they're a bit goofy, but a bit more usable than he gives credit for.
Of course, having said that, I still keep all of my tripod-shots to ISO 100. You just can't argue with the extra dynamic range. But for the hand-held (no-flash, usually) shots of my kids, 6400-12800 will do just fine.
Incidentally, one thing that has slightly annoyed me with all of the D800 reviews I've read. If you look at the DxO chart, its dynamic range drops like a rock when you raise its ISO. So while its SNR remains comparable to a D3s or D4, it quickly becomes slightly worse in color sensitivity and significantly worse in dynamic range (basically losing a whole stop of range with every full stop increase in ISO). So it's a bit misleading to say that it is equal to a D4 in high ISO performance.
In any event, here are the pictures, in order by ISO (100-204800):