Works well against many sounds, in everyday life it is most like you do not really know that it is there before you turn on noice canceling (I have not tested them by air or on bus etc).
However, there is a major disadvantage! They make almost no difference to children's screams! It is somewhat muted, but is still far too high to be appropriate for peace of mind. If that is exactly what you want to avoid, then these are not the right solution. Since you still have to have music running to drown out the noise, it works almost as well with regular headphones (though at a lower price).