I think it also depends quite a lot on the type of game or what's going on in the game at the time the frame rate dips*. If quick, precise reactions / controls are important, a dip in fps is just horrible. For e.g. adventure games (even with free movement instead of in 'steps' like in Myst), or RTS's where you may need to act quickly, but not terribly precisely due to the map being made up of a grid, or an MMO like WoW or Eve where again speed is necessary, but not really precision because you lock onto and switch targets by clicking anywhere on them, and then all actions thereafter are automatically applied to the target, dipping down to 24 - 30 won't hurt gameplay. It might be annoying to me, but it's not going to really hurt me.
OTOH, with games like First Person Shooters or Platformers with difficult 'jumping / timing puzzles' or enemies that have to be attacked in a special way to be defeated, a sudden dip in frame rate will make the game noticeably more difficult.
*This is another important thing that others have alluded to - suddenly dropping from 60 to 30 fps in the middle of a game is BAD, especially since it's likely that the reason the frame rate dipped is because the game just got a lot more difficult / complicated. However, running consistently at 30 fps on the exact same game might be playable (if still somewhat annoying). Noticeably bad fps is not nearly as frustrating as generally good fps with a few points of noticeably bad fps mixed in.