I have slowly developed the 'need for speed' over almost anything else. I so often find myself in need of light that my motto became "nothing slower than f/2.8"! Every time I tried a f/4 or even f/3.5 I ended up giving it back! Maybe it is just my experience, but if I get IS one day it will have to be f/2.8 or faster...and that means a 500 pound lens. It has been my delemma for some years now. My 70-200 f/2.8 is as heavy as I can manage with my particular handheld shooting style, eg. canoeing, rock climbing, hiking, scuba diving. Just my 2 cents. And finally at 1/15s a moving object will blur...IS or not.