Its like night and day for me - I've tested it a fair amount - I have both monitors hooked up to the same PC and I can play 10 games of COD in a row on the 60Hz and at best get half the KDR I will get on average playing 10 games on the 120Hz.
I don't actually notice it being _that_ much smoother in operation tho it is a little, but the difference in response and reduced residual image issues make a big difference to how well I do in games.
Next monitor I get will be a 120hz. Can't wait for even more cries of 'Hacker!'; always cracks me up why they moan that I use the awp on a server that...erm allows the awp.