I have a 60hz monitor and a 144hz monitor and I simply can't tell the difference in normal usage. I can see a difference on something like http://testufo.com if I specifically look for it, and until I checked that I assumed the OS was just lying to me and the monitor was actually set to 60hz.

It's odd because I can tell right away if a game is set to 60Hz instead of 144Hz when first launched before I adjust the settings. The mouse cursor just feels laggy.