I debated between the D4 & D800 before buying my D800 in June.
I have a D300 that gets 8 FPS vs the D4 at 10 fps so I wouldn't spend the money for 2 extra FPS. The low light capability of the D4 is nice but I don't often need that kind of low light capablility.
I didn't like the size of the D4 to carry all day on a travel vacation. I felt before getting the D800 that 16 mpx would be ok vs 12 on the D300.
After 3k shots on the D800 I am glad I choose it over the D4. It is really 3 cameras in one. In DX mode it is a 16mpx D300 for the 1.5 x focal length when I shoot wildlife.
When in FX mode I can crop a very small area of my image and still get high quality resolution.
I find it focuses way faster and more accurately than the D300. I find my copy of the D800 overexposes a bit in some situations so -.3 EV is a good default. I love the in viewfinder horizon that allows me to shoot a level image.
I have not had any problems with the D800 that some posts talk about and I am very happy with the image quality and the low light capability is WAY WAY better than the D300.
I bought fast Lexar CF and SD cards and when plugged them into a Kingston usb3 card reader 50+ mb images dnload as fast as my D300 images did on a usb 2 port on my older desktop. I find that when opening a 50 mb image in CS5 it takes a sec or two longer than a D300 NEF but not a problem at all.
My biggest downside to the D800 is my wife is comparing her D7000 images to my D800 images and saying wow. It's making me nervous.