[Cathus]

Unless your camera can use back button focusing then it is unwise to use AI Servo mode most of the time. (depending on what you shoot most of the time)

In the servo mode the autofocus tracks whatever is under the focus points, so that if it moves the subject stays in focus, the problem arises that if you are shooting something which isn't moving, you generally focus on the subject, hold the focus and then recompose, with AI Servo mode, as soon as you recompose the camera thinks the subject has moved and unless the original subject is still under the focus point, then it refocuses putting your original subject out of focus.

An example would be a portrait, you focus on the eye (usually using the central focus point which is the most sensitive point), you then recompose so that the eye is, say, on one of the thirds rather than slap bang in the middle, but as you change composition your camera refocuses on, say, the nose or the background.

You can't use manual on AI Servo because it is an automatic focus, as soon as you switch autofocus off that mode becomes redundant. If you turn the autofocus on the lens off, you will find you no longer have the AI Servo feature available.

A compromise is the AI Focus. This should focus on the subject if it is moving, if it stops moving or is stationery to start with the camera shouldn't refocus if you recompose. I'm not sure how successful this is at follow focusing as I never use it, but you could try it to see how you get on.
Again, if you switch to manual focus, this feature won't be available on your camera.

Back button focusing allows the best of both worlds. Some cameras have a feature which you can access in your custom functions menu which allow you to move the focusing button away from the shutter button and put it on one of the rear thumb buttons (the ones you use to change focusing point or lock exposure). You then keep your camera permanently in AI Servo mode and focus with your thumb and take the shot with your finger. The means that if a subject is moving you'll always track it, but if it stops you just focus with your thumb, take your thumb off the focus button and recompose taking the shot with the normal button. The focusing won't re-engage until you press the thumb button again. It sounds trickier than it is but once you get used to it its fine.

I'm not sure if your camera will do this. I use it all the time on my 1D MkIV. The only problem is when you go to get a quick photo and forget that the focusing is on the thumb rather than the shutter button. It is especially handy for sports photography where you would normally keep the camera in AI Servo mode to follow the action, but if you anted to take a portrait-type shot or compose a shot more aesthetically pleasing,you can focus and recompose without switching modes on the camera.