After much agonizing, I bought a Nikon D3x. After owning the D3, unwrapping and getting started with the D3x is totally anticlimactic. Everything is identical across the two cameras and instantly familiar. This is definitely a good thing when using your muscle memory and working quickly under stressful situations. I took it out, unwrapped it, slapped in a battery from the D3, and started shooting.
I took several frames with the lens I happened to have on hand, which was the 105/2.8VR macro lens. Those first frames were stunning. Out of my office window and down into the parking lot, I was able to easily resolve the expiry dates on license plates (and very nearly the smaller barcodes), and the subtle weave on a person's pair of sneakers. They had extreme sharpness and bite and the per-pixel sharpness is better than the D3, which always looked a tiny bit soft without some sharpening. Tonality, especially the crucial skin tones, seems to be extremely good. I'd also noticed previously back around Christmas when I had one of the first D3x's in Canada on loan for a few days that the blacks were pure and noiseless. I would extend that to say the D3x has incredibly low levels of noise across the board at its base ISO of 100 where it's noticeably less noisy than even the D3 at its base ISO of 200, where some noise can creep into the shadows and midtones. This smoothness is visible when bumping up the overall exposure, and makes for a much more malleable file in post processing.
Noise does seem to creep in earlier than the D3, with a little graininess visible at ISO 400 and up. Therefore you have to be more careful with exposure compared to the D3, which tolerated all sorts of sloppiness. But the magic of having all these pixels is that if you resize downwards to D3 resolution, the noise essentially gets averaged out and the results aren't too far off the D3's sensor. Now, some detail is being smoothed away by the D3x's default noise reduction that kicks in at ISO 400 and above (it can be turned off up until Hi-0.3 at which point you have no choice in the matter), but the final image does show more detail with very similar noise characteristics. The D3x stops at Hi+2 or 6400 ISO, and while the D3 goes two stop higher, I wouldn't hesitate taking the camera up past 1600 with the downsizing in mind.
I can see that the camera is going to be extremely challenging on lenses, exposing any little flaw. It'll be interesting to see what lenses fit the bill. Even the 24-70 looks like it could be challenged, and it is an incredible lens. It could well be that some decent primes are the way to get the best out of the body even though I'd pit the 24-70 up against the very best primes in its range, just as I would the 14-24. Likewise, the camera is also going to expose every little flaw in shooting technique: Every bit of camera shake, lack of use of a tripod, focusing slightly off, and lack of depth of field control is going to show. And don't think that increasing the f-stop to broaden the depth of field is going to cover up misfocusing because diffraction also kicks in at a lower f-stop and anything over f/8 is getting dangerous for sharpness; f/16 and above definitely are to be avoided.
Basically, I can say that if you don't invest in exemplary glass and technique, the D3x will be a disappointing waste of money. You'd be better off with a D3, D700 or D300.
The D3x looks like it will be an extremely competent studio camera and I can't wait. Its big draw, besides the resolution bump, was the native 100 ISO support, which helps in running larger apertures in the studio and in the many situation where studio lights can't be powered down enough and the D3's excessive sensitivity was an issue. Running full-tilt at (only) 1.8fps in 14-bit RAW mode seems pretty workable, especially given that studio lights normally don't recycle that fast (except maybe the new Profotos), and faster shooting is possible in 12-bit mode.
For wedding or PJ work where high ISO is a must, the D3 is still the champion of course, and at usual print sizes I doubt most people will be able to tell them apart. But I can definitely see using both D3 and D3x together and applying their complementary strengths. They are different cameras, naturally, and it really does make a lot of sense for Nikon to release a camera like this in a role that is essentially a viable competitor to medium format, optimized for the best IQ at low ISOs, while the D3 excels at high ISOs.
As an aside, praise goes to Nikon Canada, who recently announced a free 2-year extended servicing period for D3/D3x owners. This will take care of unlimited sensor cleanings, and one major tune-up per year. This is a nice bonus for those who have shelled out for a professional body and often need the corresponding level of support. In a great goodwill gesture, they've also included previous owners into the program by allowing the the servicing period to go to March 2010 or two years after the purchase date, whichever is longer.