Dave- this is going to turn into a whole new discussion, but its one I'd like to have-
I believe I am in a small minority that actually uses the AI Autofocus on the 40D- from what I understand, it is supposed to act like one shot, unless the subject is moving, then it acts like AI Servo. I know most sports shooters will use AI Servo, but I have some issues with that- notably that the shutter is permitted to open before focus lock is achieved- as a result, the first shot of a burst is often out of focus. Since my first shot is usually my best, this was making me unhappy. Secondly, I will occasionally lock focus and recompose- AI Servo seems to constantly refocus and I lose that ability.
To the point though- the * button.....
I tried it- I don't get it. What is the difference between that and pressing the shutter 1/2way? I couldn't tell any difference at all, and with the * button enabled, I was pressing 2 buttons instead of 1- where's the benefit?
As I do it now, I do shoot the jumpers in a burst. Of about 4-5 shots, I usually find that #3 is blurred- there seems to be an issue in that part of the process where the equipment is not keeping up with the action. The others are usually well focused though.
So convince me, please- Why do I want to use the * button? What's in it for me? 
I mean this in a respectful way- I really do want to know.
Thanks-
Steve