I used a 240 Hz PWM-dimmed monitor for a couple of years and I adjusted, but when I switched to a flicker-free one, it was very noticeable and bothersome to use the old one. Even though it's not perceptible when looking at a fixed point, when moving one's eyes around the after-images are easy to see. Even 1000 Hz PWM-dimmed LED strips are easy to notice when looking around the room. The light is basically being panned across one's retina like a panoramic camera/oscilloscope, logging its brightness versus time.