I have just bought a D800, this are my initial impressions
Contrary to what seems the situation in the USA , getting it here was fairly easy, 1 phone call to a local distributor and I had it the same day. Guess there aren't many photo crazy people in México.
I'm coming from a Nikon D300 and my comments are based on my experience with it vs the new D800.
I bought the D800 looking for an improvement in high iso , and dynamic range, which it certainly shows, there are other advantages, and some disadvantages,
Why the move to FX? because I got tired of waiting for a DX improvement in a body similar to the D300.
I never bought into the D7000 because I felt the improvement from the D300 was marginal, I think the same is when comparing the D700 to the D800, this are MY conclusions from seeing reports as the ones in DXOmark.
About using the camera:
Usability is very intuitive moving from the D300 to the D800, biggest change is the new AF control, but it is very easy to adapt from using the tiny lever on the side of the D300 to using the button and rotating knobs on the D800 (similar to a D7000)
When holding the camera, the grip is smaller in the D800, let me explain: with the D300 I can have it hanging from 3 fingers , it clings to my hand even if I do not use my thumb , I had removed the camera strap from the D300 and never felt insecure. With the D800 this is not the case, the grip is shallower , maybe a 1/4 of an inch but I do need to use my thumb to hold it otherwise it will fall from my hand, hopefully someone will come out with a replacement grip in the future, (and i do not mean the vertical grip, replace the rubber in the front for something a bit more bulky) the D300 and the d700 feel really nice in my hand , the D800 doesn't, now this "hanging" from my fingers is only while walking around holding the camera , for shooting the grip is good and I do not feel it makes a difference.
Getting sharp images is not more difficult than with the D300, hold the camera correctly and images come out great, use a sloppy technique and it shows
Autofocus is certainly improved over my D300 things just snap into focus, to me it seems instantaneous, and have not have a situation of hunting for focus even in dark situations.
I tried Auto area continuous autofocus mode on one of my kids in a swing, no problem at all, and no it did not select idiotic spots like the unmoving frame above the swing, it focused on the face. Still have to try with a BIF, that will be a more practical test.
Shooting liveview the AF has to many options , looks like a fancy point and shoot and I do not care much to explore all those options, the viewfinder is way more attractive. Except for macro shooting an then I focusing manually, in this situation Live view is awesome, I used it while shooting from a tripod, I'm not sure if the mirror moves or not when taking a shot , there is certainly a number of noises and certainly takes some time for the display to come back on again, the results are SHARP, even when shooting at 1/6 sec, there is no vibration.
Full frame DOF is wonderful ! Even if not shooting Wide open, there is some blur behind the focused areas which looks really nice.
Dynamic range is also great, all that is said about how better is the sensor so far seems true. Did some shooting in bright sunlight with mixed shadow areas and I have no problems with highlights or shadows, very nice, easy to work with the raw files, or the SOOC Jpgs are still really nice, I'm very pleased with this aspect. Metering seems to need to compensation as it did with the D300, I rarely shoot at 0EV, it was more often at -0.7EV.
More noise due to higher megapixels? Not what I have seen, specially when compared to the D300.
Disadvantages, certainly I have just 3 FX lenses... My DX lenses work fine in DX mode and I still get 15MP images, but the viewfinder only has a rectangle indicating what is in the DX frame , I have not used it enough to judge how difficult it is, certainly useful until I replace the DX lenses , that is if I decide to replace them all.
I'm selling the D300, I do not need 2 DSLRs
As for FX right now I have the 50mm f1.8, 150 f2.8 macro and the 70-300 zoom,
So I'm not covered on the wide side, I'll eventually like to get the new 28 mm f/1.8
and maybe later a fisheye lens, considering the sigma 15mm f2.8 , mostly for the shots I do at night from the stars
Getting the pro f/2.8 zooms is not something I'll like to do, they are huge.
Another disadvantage, the RAW files are 40ish MB they will use 3x times more disk space compared vs the D300. Processing them is a bit slower but only a fraction, for me that I process a few images at a time is not a problem, I'll get up to get something from the fridge and the processing will be complete.
There is still plenty of stuff to learn but so far I'm very pleased with the camera