30 vs 60 fps - can you really see the difference?

[SuperNova]

I believe I said this before, but I personally don't see a difference (at least not without major concentrating) between a locked 30 and 60 fps, BUT I feel it while playing.
I do see diffrences between 24 and 48 frames so I must be able to percieve higher framerates, but once it gets over 30 it becomes very hard for me to see.
Now while actually playing it makes a world of diffrence.