I use back button (AF-ON) focusing in AI Servo mode, exclusively for the majority of my shooting (landscape, wildlife/birding, auto racing (NASCAR, NHRA & ALMS), architecture, people, street, travel, etc.). Using AI Servo and back button focusing, allows me to track moving subjects (holding the button down) independently of shutter release, which I find results in faster and better results (more spot-on focus). For static or slow moving subjects, I just press the button to gain focus and release it to lock focus. This works really well in recomposing the shot while maintaining focus and eliminates having to select other than the center AF point or points (expanded or zone). I also use the Auto AF point selection method in AI Servo on some moving subjects, allowing the initial AF point to continue to track the subject across the frame. The back button/AI Servo method allows me to switch between one shot and AI Servo instantly, without having to dedicate another button or make changes by changing camera settings. I was hesitant at first to use this method. However, now that I ues it full-time, I wonder how I could ever do without it! My digital bodies are as follows: EOS-1D X, 1D4, 5D2, 5D. Film bodies: 2x EOS-1V and a 1n. My Lenses are: 20-35, 24T/S, 24-105L, 28-300L, 50 1.2L, 70-200 2.8L, 70-300L and 100-400L. They (lenses) all work excellently using the AF methods I listed above.