But you know it sometimes does not, Jon, per my posts on this very subject a few weeks ago. It can cause blurry images when on a tripod at longer exposures, but the manual says it should not matter.
I've already addressed that several times in my previous posts, including my reply in that thread. On a tripod, exposures longer than a second or so, IS is bad, because of the "drift" behavior. For exposures faster than 1/8 or so where the "drift" is not a significant, IS offers a clear benefit, especially if there is wind or some other factor that is causing shake visible in the viewfinder when IS is off. For tripod exposures between 1/8 and 1 second, IS may or may not help, depending on how much shake you have on the tripod. It really is that simple.