There is some good news out there, though. Have any of you seen the recent series of sample images? I thought for sure someone was going to post the link because they have been there for 36 hours or so, but I haven't noticed anyone mention it yet.
If you toggle through the photos at full screen size (not 1:1), it's impossible to tell any difference until beyond ISO 12,800. At 1:1 it is great, no worse than ISO 400 on cameras five-six years ago. The top of the regular ISO range, 51,200, actually does look usable as well--certainly better than 25,600 on the 5D3.
I am so excited because I was still grappling in my mind with the worry that I should have gone with the D4 (I already own complete systems of both Canon and Nikon, so the array of lenses isn't an issue for me).
Hmm, I guess I would disagree that there are imperceptible differences till *after* 12800. At full size, it is readily apparent that there is some pretty heavy duty noise reduction going on at 12800, and its even visible at 3200. Below 3200, the difference between ISO 100 and ISO 1600 is largely imperceptible at 1:1 crop.
When viewing the images scaled down to "fit on screen" (2560x1600 30", a tad less than 1/4th the native image size, so approximately 2x downscaling), there is minimal perceptible difference between ISO 100 and ISO 3200. At ISO 6400, things start to look ever so slightly "muddy" compared to ISO 100...fine details start to dull...although things still appear sharp. Fine detailed highlights in particular start to fade at ISO 6400. At ISO 12800, there is definite "muddying" of fine detail...the difference in the blue feather and thread wheels; the highlights of the silver bristle holder on the brush, the strainer, even fine highlight detail in the crumpled ball of foil; black printing on all of the highlight markers; finer detail in the queen playing card (which isn't really that "fine" overall)...all soften visibly between ISO 100 and 12800.
I would call every ISO setting up through 25600 "usable"...however if you need fine detail, 3200 seems to be the limit (most fine detail, including highlight detail, is preserved up through ISO 3200.) I would call 51200 usable in certain circumstances, however it definitely obliterates finer details. If I wanted to make a recording like the one NASA Astronauts made from the ISS of earth and the auroras at night, I would say that 51200 would do a better job than the Nikon D3 did on their first video, preserving even finer earthly details.
It is a bummer all of these photos are JPEG's though. I would really love to see how the same photos fare with RAW and some more meticulous, manual noise reduction and sharpening. I wonder how much detail could be preserved.