if your gpu is capable of delivering 120+ fps, then the gameplay will be smoother. You will notice the difference, no doubt. If your gpu cant do 120 fps, then dont bother, because you wont be able to enjoy the full experience of such a monitor.
I should say this... before i had 144 hz monitor, i didnt think there will be much difference, but after having it i can't go back.
the 3d difference between 60 hz and 120-144 hz is muuch more observable indeed.
PS : professional games often suggest having as many fps as posible, even by lowering quality. they would not recomend 120 + fps if they had a 60hz monitor.