I have used quite a few Ai or Ai converted lenses on my D800 and they stand up to scrutiny very well, just as LM said. I know of at least one person who swears by the old 5cm f/2 Nikkor-S (not H) 9 blade version, I have the 7-blade version but not converted to Ai, so can't test on FX yet. He also swears by the Nikkor-O 35mm f/2.
So far I have tested E75-150, Ai80-200/4, AiS80-200/4, K24/2.8, Ai50/2 on the D800 and I am pleased with the results.
I have used the 5cm f/2 Nikkor-S, 10.5cm f/2.5 Nikkor-P, 50mm f/1.4 Nikkor-S and 105mm f/2.5 Nikkor-P.C. only on DX. So have to wait and see how they perform on Df.
I believe the old glasses are all good.