The IS should be a very nice feature for those who shoot spiders, frogs, and such in the forest where often one can't use a tripod. Some spiders and frogs won't stand still waiting for you to set a tripod. However, IS may be very useful when not shooting macro, since 100mm f/2.8 is a great telephoto lens.
There is a guy who posts in this forum (dllavaneras, I believe is his name), who photographs insects and other subjects in the Central American forests. He often does not use a tripod, and may be able to tell you why in more detail.
Some of his photos:
http://dllavaneras.deviantart.com/