Jim,
This guide by Les Zigurski is interesting.
http://www.wildlifeimagesbyles.net/T...iii_guide.html
He uses the high sensitivity mode. I tested my 1DII against my dogs playing fetch at close range. They are faster than the AF system can handle so I was able to test again and again and found that for my style, the high sensitivity mode worked best. I generally keep the shutter button half pressed and let the AF system tract the subject. If the AF point slips off the subject, even with a complex background, I find that the high sensitivity mode re-acquires the subject fastest and that has saved many an action image for me. I find this is particularly good when the subject suddenly changes directions.
Roger