I hand hold my 500mm quite often. I have a Sigma 170-500mm, not nearly as heavy, fast, sharp or expensive as the Canon L series, but hand holding is possible. I usually have to boost the ISO setting to get the shutter speeds fast enuf and IS would really come in handy in situations like this. I would prefer to shoot with it using a tripod and cable release, but sometimes it's just not possible, and IS would really help out.