I'm in my 4th season of shooting motocross racing, shooting 18-20 races at 5 different tracks over the course of each season. On average, I shoot approximately 2,000 - 3,000 images per race.
I shoot almost exclusively with my 70-200 f/2.8L IS II on a 1D mark IV or 5D Mark III. I shoot primarily at high shutter speeds to freeze the image as best I can to capture the bikes coming off jumps, doing whips and scrubs and churning up rooster tails in the tight corners. Normally, these shutter speeds are anywhere between 1/2000 and 1/3200, depending on prevailing conditions, but can be as low as 1/640 and as high as 1/4000. I NEVER
turn off the IS and have NEVER
seen any negative effects.
I occasionally drop to a very low shutter speed to do panning shots when I want the subject in sharp focus and the background blurred to give the feel of speed. When I do, I set the IS to Mode 2.
In summary, the vast majority of my images turn out critically sharp and I see no reason to ever turn off the IS.