Even scrolling text or dragging windows around at >120 Hz is a game changer. Going back to the MBP display gives me instant headache until I re-adapt. It's hard to describe, it's like all of a sudden everything has a very distracting input lag.

Now I just need to wait for a 4k 144Hz IPS that can be driven via MBP (or just get what's on the market and run eGPU)