Image Stabalizer ON/OFF Test - 1/500 seconds
TOP ROW: IS ON
BOTTOM: IS OFF

I ONLY took six shots IS ON and six shots IS OFF and posted them ALL right away. I did not select the best images to prove a point either way.

NOTE: The left smaller images are at f/6.3, the larger images on the right are f/5.6

CONCLUSION: The IS makes a big difference for me at 1/500s.
SIDE NOTE: For me, the IS makes center-point one-shot AF more accurate at all speeds