as with all the other comments so far:
they dont isolate fully, infact no where near. If i have no music playing, and have them in, i can hear a fair bit, its just toned down in loudness. I guess you could liken it to having the door to your room open and closed. With it closed, you still hear sounds, but they are more muffled and quieter.
When i have music playing, i think even at a quiet volume, most outside noises disappear unless they are very loud. eg: when in the tube, i can still hear rail/wheels grinding, however its much more muted.
In summary, for me they isolate almost the perfect amount, with music playing at a low volume, its enough that you wont hear everyday background noise, however you will still hear something like a car horn or someone screaming.