I've been shooting the D3s and the D800 back to back now for several months. But I am tainted by the fact that I shoot mostly sports with the D3s. I don't use it for much else. At ISO 6400, my personal feeling is that the D3s is doing a better job. At 3200 it's close.
If I wasn't shooting sports, I'd say the D800 would be sufficient for my needs, or frankly for most people's needs. Focus is great, files are terrific, DR is astounding. Nikon got a LOT right with the D800. But you do have to compromise SOMEWHERE, and for me, it's the high-ISO noise. Remarkable for what you get, but it wasn't built for that purpose.