The D800 focus system is great !!!
The high resolution harm severely the contrast and it looks some times as not sharp enough.
Adding contrast and sharpening is essential !!!
All together, I find the D800 a good camera but more difficult to handle than my old D700.
The Sigma 15, Nikon 60 and Nikon 105 vr are really good on it as John says.
I'm sure the D4 will be much easier to handle !