It really depends on the game really. South Park was fine at 30fps because it didn't require quick,decisive action. Need for speed rivals however, 30fps wasn't enough time for me to react to the action on screen. Think of all of the minute adjustments that you make with the wheel as you drive. If you cut half the speed of what you were seeing on the road, you would be swerving like a mad man. It's said that fighter pilots are able to see up to 255fps. It really does depend on the action of the game, and what your reaction time needs to be.
I prefer 60+fps. I may be bias because I've been a PC gamer, after the SNES.