I hardly notice a difference between 85Hz and 100Hz, although it is definitely there. I can't see 120Hz being much better, but I have never tried it and could be pleasantly surprised. There is quite a difference once the refresh rate drops below 85Hz. 75Hz starts to get to me after a while, but it does take an extremely long session of computer usage.
As a side note, I was forced to take driving school to get my driver's license. I was told, in class, that each eye captures frames individually at a rate of 30-40 frames per second. That's another reason that makes me think 85Hz is around where the threshold is.