Although it's not quite as fast as its older sibling the D3, the D3x still has incredible performance by any other standard. In high-speed continuous mode it can maintain over five frames a second for 21 frames even when shooting in 12-bit Raw plus Fine JPEG mode, a combined file size of around 40MB. It is possible to slow it down by switching to uncompressed 14-bit Raw mode, but even then it can maintain 2fps with a combined file size of over 52MB. Needless to say that in single-shot mode it can shoot as fast as you can press the button, with a shutter delay of less than 40 milliseconds.
As I've already mentioned, the autofocus system is superb. Once it has focused it can track moving subjects even if they change their distance to the camera or pass partially behind other objects. Without the assistance of a separate dedicated flashgun it does falter in very low light, hunting backwards and forwards for a second or two, but it will find focus in surprisingly dark conditions. In good light it is the fastest autofocus system I've seen on any DSLR so far. It's hard to see how anything could be faster without using de-focused temporal perception, which is of course impossible.
One criticism that I have seen levelled at the D3x is that it has less dynamic range and more high-ISO noise than the D3, due to the more crowded sensor. I don't have any images from the D3 for a direct comparison, but it seems to me that this is nit-picking for the sake of it. There is a faint grain of mid-tone noise at 800 ISO, and some tonal gradients aren't as smooth they might be, but colour rendition is excellent and while there is some loss of high-frequency detail it's fairly minor. The results are as good as or better than any comparable camera that I've tried, including the Canon EOS 5D MkII and the Sony Alpha A900. I'd certainly be happy to use the D3x at 1600 ISO for professional work.
In all other respects the image quality is as near perfect as it's currently possible to get. The level of fine detail produced by the huge sensor and a typical top-quality Nikkor lens such as the 24-70mm f/2.8G ED seen here is simply astonishing, far surpassing any APS-C DSLR. The only way you're going to get anything much better is to mortgage your house, and possibly a couple of neighbours' houses as well, and move up to a digital medium format camera. If you're planning to do that, can I have your old D3x please?
It pretty much goes without saying that the Nikon D3x is an incredibly good camera, with the kind of performance, rugged reliability, versatility and most importantly image quality demanded by the top professionals. It is big, heavy and quite breathtakingly expensive, but if you're the sort of photographer who needs to have the very best then ask the picture agency you work for to buy you one.