I've been using my on mode 2 since I got it, as you say for panning but. . .
Yes for prop stuff where you may be at the 300-400mm focal length and you want to get that shutter down below 200th sec range for prop blur. Of course when in the shorter focal lengths and you want to get to that lovely 1/15th for a lovely full disc blur. For jets as long as light is available I push to be minimum One stop down and get that shutter speed up around the 800th plus range to prevent any camera blur through shake. I used to use it on everything but can't see the use of it at the higher shutter speeds.
The IS is definately good for 3 stops once you tweak your style of holding and shooting though.