I just got out for about 4 hours of serious (landscape and macro) shooting with the D3x this morning, and have been looking at the files for most of the afternoon. The easiest comparison I can make is to the 1Ds mk II (note: NOT mk III), as that is the highest-resolution camera I have a lot of experience with (other than the D3x). I am comparing at ISO 100, converting from 14-bit NEFs (in Nikon Capture NX2) and viewing at 100% on screen (unfortunately, my printer is 180 miles away right now, and I'm on my laptop monitor, NOT my calibrated work monitor, because I am visiting my parents for the holidays). There are two apparent differences - first is the incredible sharpness of the D3x. When I've nailed the focus, the D3x looks very darned sharp at 100% without applying any sharpening - because of the AA filter, the Canon never did that. It would PRINT very sharp, but 100% on screen always revealed a slight blur. There is absolutely no noise in an ISO 100 D3x file, even at 100%, which adds to the impression of sharpness - very slight shadow noise in the Canon files adds a slight haze to dark ones - that is simply not there in a D3x file. The second difference is the dynamic range - the D3x has about a stop more range in the highlights, plus at least an extra stop in the shadows, maybe even 1.5 stops extra in the shadows, all of it very clean. This camera, properly handled, should print 24x36 inches with ease from its base ISO of 100 (I get 16x24 out of the Canon, but don't like to go larger than that).
ISO 400 on the D3x is very usable - it looks roughly like an ISO 100 file from a 1Ds mk II in terms of noise - it may have extra dynamic range, which I wouldn't have seen because my ISO 400 tests were on a very dreary, grey day and would have fit easily within the DR of the 1Ds mk II). This is comparing on a per-pixel basis, so the D3x file still has 3/2 the detail in it, due to the increased resolution... I have even fooled around a bit with ISO 3200 (HI 1.0), which looks awful on screen (although quite good considering that it's ISO 3200 - much better than any ISO 3200 film ever looked), but will make a pretty decent 8x10 print with no trouble, and should even print 11x17 with some careful handling. I didn't buy the camera to shoot at ISO 3200, but it's nice to know the capability is there should it be needed. Those unbelievable ISO 100 files are what I bought the camera for, and it is certainly worth its price for its low-ISO performance! There is something highly unusual in the imaging chain of the D3x to get these results - the sensor may NOT be stock Alpha 900 issue (I suspect it isn't - I don't have a lot of Alpha experience, but the test files I've seen are not anywhere near as clean, even at low ISOs) , and if it is, the AA filter in the D3x is extremely unusual, probably made out of pure unobtainium.
Add that performance to a superb rugged and ergonomic camera body with class-leading AF and metering, and the result is a remarkable machine. Yes, it's expensive, but the only way to get better files is three times as expensive and not nearly as rugged.