I got the D700 when it just came out, and I have always been getting outstanding results.
With my Zeiss zf lenses, I routinely use LiveView focusing. Since the LiveView implementation is better with the D800, I acquired one. So I now have the ability to directly compare captures of identical subjects by the two cameras. When viewed on my calibrated 30" monitor, the results are pretty much the same in most cases, though the rendering by the two cameras is slightly different. Also, due to the higher DR, the D800 makes it easier to recapture highlights.
Interestingly, my Nikon 24-70/2.8 shows some softness at large apertures with the D800, while its performance with the D700 is impeccable. Conversely, the results with my Zeiss primes are significantly better on the D800 with respect to color and micro contrast.
I continue using my D700 with my Nikon zooms, while the D800 is dedicated to the Zeiss primes. In summary, unless you want to print very large, need video, better LiveView or only use the best lenses, I see very little merit in "upgrading" from the D700 to a D800.