I found that image stabilization makes a huge difference for me. In some specific situations, I was blown away.
I took some pictures at a concert in particular, with a 70-300 IS mostly set at 300mm (which we've been talking about here on the BBS a while ago). Instead of the expected blurry mess :), I got some amazing pictures. That is entirely due to the good equipment and IS in particular, because I really did not do anything special, I had no tripod or even monopod, and I was essentially lacking any experience in shooting in those lighting conditions. I just gave it a try. Those pics were so successful among friends I ended up putting them on Flickr for them to download!