This web page gives an excellent example of what the difference is.
15fps, you notice looks a bit odd.
Compared to 30fps, it looks really smooth and nice. 30Fps looks great.
When you hit 60fps, when you compare them you again notice that there is a really smooth feeling to it.
30fps is decent, certainly playable, but when you've got '*** hits the fan' action going on, having twice the frame-rate is where it REALLY makes a difference in playability.
That being said, having FPS jumping between 30-60fps is in my opinion worse than a solid 30fps. Having 60fps is only important if it can be kept consistent.
o yeah... i also feels that. especially gtr evolution. sometimes, the fps bump from 35 to ~55 fps, the games smoother. but when thats happens, the game lag for about 0.1~0.25 sec (freeze in blink of eye, but noticeable). even worse when going down from ~55 fps. kinda disturb me. well, only happens to this game lately. other games (like nfs prostreet) are not that appearing (especially when running below 25fps, called 'unplayable' like cod4, codmw3a, nfs shift 2)(shift 2.. trust me, like 10~15fps even at lowest setting...)