If you'e shooting 12 fps sports, panning around after the fast moving action, the first few frames may be softened as the IS stabilizes the image in the frame.
I see it referenced by people in IS discussions - but I don't own an IS lens to test the theory.
The only use for IS I had considered with my long teles, was the panning mode IS when doing speed blurs on cars at slower shutter speeds.
I could see that mode getting you a higher keeper rate - and have noted that the number of such shots used in the motor press has increased post IS being released...