IS reaction time - is IS fast enough?

photostarts wrote:

Earlier this week a salesman in Jessops gave me the "hard sell" in trying to persuade me to buy the non-IS version of the 70-200mm f/4L lens. I guess he thought I'd find the price attractive, as the version with IS costs a whole lot more.
"But it gives you up to 4 extra stops," I pointed out.
"In what way?" he replied. "It's still f/4, so you won't get any benefit."
I explained that having IS meant you could stop down or use a slower shutter speed.
"No, no," he said, "By the time the IS has done its job, you'll have missed the shot."