To me, IS helps me in many situations when tripod / monopod is not available:
1. When I'm zooming in deep and slight breathing, heartbeat (I'm skinny... so when my arms are pressed close to my body, heartbeat can sometimes result in very slight vibrations......), hand shivering - all these can be saved, to some extent, by IS.
2. When shooting in dim light conditions and flash is not an option. If shutter speed continues to be slow even after opening the aperture and using high ISO, then the IS can be a life-saver.
As what others have mentioned, although it also depends on what you're shooting most of the time, it's better to have IS than not have it.