Here's a few photos on the longer end of the 24-105 mostly to demonstrate the handholdability (a new word?) of the lens with IS on. Like Bob said, it will save you when you're shooting without a tripod.
1/8th, 98mm @ f/4, ISO1000, IS on, handheld
1/8th, 67mm @ f/4, ISO800, IS on, handheld
1/10th, 102mm @ f/4, ISO1600, IS on, handheld