IS is very usefull for f/4. I am upgraded my non-IS 70-200 f/4 to IS version because of quality @f4... Now I/You can not to stop down at > @f5 to get extreemly sharp images.
But one dissapointment is - IS noise. But it does 4-stop ... what else can i say.
Actualy also i found focus/zoom rings more tight (better IMHO!)