The 120 Hz displays are very noticeable when side-by-side. For example, my iPhone switches to 60 Hz on low power mode and it suddenly feels like it's a fast slideshow instead of "smooth".

There is also an adaptation factor: Good performance is only noticeable in contrast to poor performance. You get used to it very fast and stop noticing, unless you go back to the old system which suddenly feels "broken", even though you may have considered it perfectly acceptable before.