It is absolutely lethal when you catch somebody by surprise with it. If they have seen it before, it is possible to nick the pawn and still ward off most of the attack - black has to play very creatively to give himself chances.
One thing that is always true is that white has to halt all ideas of a kingside pawn rush as there is simply not enough time to execute it (although I did once get somebody pull it off in a rapidplay game by allowing me to trap his bishop with my oncoming pawns on the queenside but by the time I had won it, it was too late and his attack had materialised - although I was a bit complacent and missed a good move to neutralise it over the board)