Recently moved to a D800 after a few years with my D90.
I had a 50mm 1.8G and the 105mm VR for full frame, but had not decided which FX midrange zoom I would put on my wish list. I did have the 18-200mm DX, which gave me nice sharp results on the D90, and I decided to try it on the D800 one day so I didn't have to lug the D90 around as well.
An immediate downside is that the useable viewfinder is reduced to the centre only with the automatic crop, making it difficult to frame some shots. However, even with the crop factor, I gained a bit of resolution on the D800 (15 versus 12 MP)
But what I noticed immediately were the wonderfully detailed and sharp images using the 18-200. Using the raw files in LR4 I could pull what seemed to be a blown-out sky back so that it looked perfectly exposed, and recovered detail in dark shadows. So there may be a benefit to using the DX lenses on your D800 just for the better processor and dynamic range. I was stunned. So stunned that I somehow found some cash to pick up the 24-120 f4, a lens that is giving me great results.
Jim