i think some of you might have misunderstood
we all know 120hz is need for 3d gaming for the shits and giggles, so lets skip that
you might say human eye can only see 60hz, but there is a huge difference between 60hz and 100hz that will definitly feel it.
i mean if you are not that serious about playing a fps game, 35+ fps is good enough for you which means you have a ok video card and a 60hz monitor
if you are seriously about fps gaming then buy the top of the line video card and tweak your setting where you get 100+fps constantly, heres where your 120hz lcd comes in.
when you swipe your mouse quickly across your screen as you are turning fast in game, you eyes will be able to pick up more things than if you have a 60hz lcd, meaning you will spot people a bit more clearly.
thats if you are really that good and have the reflex, i had a samsung 226BW, then upgraded to a acer gd235, i mean, sure lcd is a lcd, its only slightly better, but when you are at that level of gaming, every little millisecond counts, so to me, 120hz with 100+fps in game, is a huge difference, to some others? might not be as much.
i will find bf3 as unplayable at 60fps, with gtx590 and lower some settings i can get 100fps minimum as the graphic doesnt matter to me. but some people will think running it at 30fps is pretty good so i guess i can tell you there is a difference, but does it worth the money? its really up to you