I generally don't use AI Focus. I like to use "one shot" mode, then hold the depth of field preview button to engage servo mode.
I used AI Focus on my T1i when I first got it - it sounded like the best of both worlds. I quickly learned that the camera wasn't very good at deciding when a subject went from stationary to mobile. Interestingly, AI Focus isn't even an option on the 1D X (nor is there a green square mode).
One Shot with a button press to switch to AI Servo works well - I have my M.Fn2 button (second button next to the DoF Preview) set for that. However, I do wish it would toggle Servo on/off with successive presses, rather than only activating Servo while the button is held down.
Another option is to use back-button AF - in Servo mode, focusing stops when you release the AF-ON button, so you can just tap that button to focus on a stationary subject (although you don't get a confirmation beep/light), or hold it down for a moving subject.