Can't say I agree with what has been said about IS and action shots. I have the 100-400 IS L and and I always leave the IS on when shooting surfing etc. I shoot at 400mm (640mm effective on 20D) handheld and get great results with IS on. The proper way to use IS is to push the shutter button halfway (pre-focus) and let the IS catch up to your motion. Continue holding the shutter button halfway, and after about a second (you can hear the IS working) then you push the shutter button the rest of the way to take the shot.
If I am shooting action, I hold the shutter button half way to pre-focus on the subject, follow the subject, and then at the height of action I press it all the way for the shot or press it and hold it down for a series of shots.
"Shutter lag" etc is not a problem, but to get the full benefit of IS, you should not just point and shoot (press shutter button all the way at once).
If IS is available as an option on the lens you want, I would get it.
- darren
I know I've kept it on during some action stuff and had no problem, but I kept wondering if it'd be better without it. I remember to turn it off sometimes but I've never done any sort of testing to see if it really makes a difference. Does IS show up as on/off in exif? That'd be good if I could go back and look at my old shots to see if it mattered.
Okay, back to work now...

. I've just been so out of it lately nothing is registering these days.
