Having tried both the D800 and the D3X for several days before deciding, the D800 has the resolution advantage and better hi iso. What I'm not liking on the D800 is that the images looked kinda digital - altho it is accurate. The white balance on the D800 is also weird. Sometimes images has bluish or greenish tint on it whereas the D3X is always spot on. What won me over to get the D3X is the film like color, tones and micro contrast and I rarely shoot above iso 400. The D3X grip gives me that happy grin all the time, while the D800 grip gives me cramps. I got my D3X for $2800 used and while the seller didn't descrbe the camera condition accurately on the sale, it was in near mint condition.
Ive always wondered about getting a D3x instead of the D800. I can't imagine there being a downside to the D3x. I pesonally won't even start really thinking about it till I know for a fact that there will not be a D700 replacement. A D600 is not that camera.
The D3X grip gives me that happy grin all the time, while the D800 grip gives me cramps. 
I got my D3X for $2800 used and while the seller didn't descrbe the camera condition accurately on the sale, it was in near mint condition.