There's more to it than raw FPS. I'm convinced some people's brain cannot distinguish between 30 and 60 fps the same way some people can't tell green from red. But that aside, there are two other aspects most people SHOULD be able to see.
1) touchscreen sensitivity and finger tracking ability. This should be night and day. To test, long press on any icon and swish it around on the page. Note how close it remains to your fingertip. Swish slow and swish fast. On stock kernel, the icon is trailing miles behind your finger. Same thing goes for finger-scrolling menus and lists. The tracking improvements make the phone feel SO much more responsive, and that adds to the perceived smoothness of the phone.
2) random stuttering not related to slow framerate. These occur all the time with the stock kernel, and very infrequently with the fps uncapped.
I've made a video that I believe demonstrates these points clearer than any previous video made to show the same thing. And if you have the eye for FPS, you should also see that scrolling through the homescreens is much more fluid with the custom kernel.
YouTube - HTC EVO: 30FPS uncapped w/ custom kernel - comparison w/ stock