Under what situation would the IS be used for?
IS can be used whenever the subject is not moving to set a slow shutter speed and avoid blur from movement of the camera. This works well in low light.
Under normal lighting conditions it will also help eliminate any blur.
However, if you want to freeze motion, you need a fast shutter speed, usually quite fast. IS does not help freeze a moving subject.
For normal every day walk around images it works very well indeed. But for sports and flying birds, you will need shutter speed.
In some ways, it can be like gaining 2-4 stops of light.