There is only one correct frame rate, ever: the vertical refresh rate of the display, with software-generated frame updates synchronized to the hardware's blanking interval. Anything lower is a significantly inferior compromise. (Anything higher is impossible with vsync, and a waste without.) Given that the quality of each frame has been pushed beyond the ability to display 60 of them per second on HDTVs, settling for 30 of them instead is the natural compromise. 30 is the highest integral fraction of 60. Miss one vertical blank, display the next, repeat. The problem is that it produces the worst possible jitter artifact. That's because the screen is strobing in your face twice for every motion update. As your eye tries to follow the motion, it appears to vibrate back and forth. Your eye is scanning across the screen smoothly, locked on whatever moving feature interests you, be it text you're trying to read as it scrolls by, or a pretty picture, whatever. But the frame updates are not in sync with your panning gaze. (This is such a bitch to explain.) The motion freezes in place half the time, causing what you're trying to look at to be in the wrong place half the time.
A higher frame rate is of course preferable. But even a slightly lower frame rate is preferrable (though not by much, since once you get below 24 or so, there aren't enough updates for convincing motion). Either way, you'll get more stable looking display on pans, less fatigue, less headaches. 40 is better. 25 is better. Even if you have to drop the vsync. 60 is best by far, but tell that to the static-screenshot whores.
Edit: Ironically, an older-tech LCD display, with slower response LCDs, would make 30 fps look better. You'll get streaking or blurring instead of jittering.
