Yeah, the IS works but just not as well as the newer version of IS. I have the 100-400 and am VERY pleased with it. Did you hear that www.canonrumors.com has something saying they THINK the 100-400mm has been discontinued... hmm... maybe a new version or a different lens that covers a very similar focal length? Can't wait to see! in the meantime, I love my 100-400! Especially at the zoo!!!