Personally, both 50 and 60 Hz are horribly flickery. Back when I used a CRT I couldn't stand anything below ~85 Hz and would get headaches. Now that the screen no longer flickers, I guess in terms of motion fluidity there's a perceivable difference between 50 and 60 Hz. Something I personally cannot unsee, though, is 2:3 pulldown, which is something you don't see in Europe (at the expense of slightly faster movies).
I’m pretty convinced on the theory that some people see “faster” than others. Most people can be taught to notice flicker, but I really feel like some see more naturally.
