Well, if you're a D3/D3x user you'll probably appreciate the feel of a real camera body vs a half camera body ...
It's true that the D800 technically surpasses it on almost every front, it isn't a dust monster which the D3x certainly could be, and is smaller & lighter. However, there's something about the sense of permanence and reliability about the bigger D3 series bodies that I feel that Nikon have lost.
Now the question is whether you are comparing used D3x bodies to used D800 bodies. If that's the case then I suspect it's as much a factor of supply and demand vs the other traits. Basically there are/were a lot less D3x bodies out there if only due to the much higher initial costs.
+1
The D3X is still the highest megapixel body with that body style. There's also another fact that is easily forgotten:
The D3X is the same generation of camera as the D3/s, D700 and D300/s, cameras used by a large number of professional photographers still. All these cameras can use the same battery, the EN-EL4a (D300/s and D700 in the vertical grip) and the same CF-cards (the D810 only accepts a few, selected cards, typically Sandisk and Lexar, and the second slot takes an SD card). While this may seem like minor thing to many, it isn't when shooting with multiple bodies at large events, like sports events that can last for days. The last thing you want to worry about then is which battery and which card goes into which camera, not to speak of body failures that could make some of the batteries I rely on unusable for the rest of the event.
Remember also that a D3X is smaller than a D800/810 with the vertical grip, and the battery door never springs open when brushing towards another photographer or object with the danger of breaking it off. This happens to me regularly with the smaller bodies at sports events with many photographers where the fight for position often can be just as hard as the fight going on at the track.
I was looking at the D3X as the most likely upgrade myself, until I realised what a great camera the D810 had become and got a great deal on one. Now, I consider swapping my three older bodies for a second D810, partly of course because it's a great camera, but also because I don't want more complications than strictly necessary. I hesitate because of the loss I'm taking on the old bodies that still work very well for me at events etc. Also, my D2Xs, like the D3X, is more comfortable to hold during long shootings than the D810 due to the shape (thinner body), weight distribution and integrated vertical grip.
Interestingly, the D3X is still a current body, and I wouldn't be surprised if some photographers still buy new ones at the street price, which seems to be somewhere between $7,000 and 8,000. It's a lovely camera, with ergonomics to match. I would still love to have one
