I have just recently switched my camera to back button focus (BBF) and I have been trying to practice with it every chance I get. I had heard a lot about it, but never really understood why it would be beneficial to have another button that I needed to deal with in addition to taking pictures of my two young kids who never stop moving! It finally clicked when I read a post on the Clickin Mom's forum. I had been having a hard time getting a really clear focus on my pictures, so when I saw this post about BBF and kids, I had to read it. The example used was like reading about photographing my kids, fast moving subjects, going in every direction.
If you don't know what BBF is, it is where you separate the focus from the shutter. Most of the time, you just press the shutter button down half way to focus your picture, then you press it all the way down to take the picture. With BBF, you press a button on the back of your camera to focus, then you press your shutter button to take the picture. This really becomes important when you are taking pictures of moving subjects (especially if they are moving toward you or away from you). Once you get your subject in focus, you can continue to hold down that back button while you pan your camera with your subject, it will continue to keep that subject in focus. This is great for those times when you want to take a picture of your toddler who is walking toward you or away from you, and if you have a toddler, you know this happens ALL THE TIME!
This also allows you to lock your focus in one point. You get your focus on your subject, then you release your thumb from the back button and the focus is locked. You can recompose your picture (without changing your distance from the subject) and then snap your picture without losing your original focus. That allows you to keep your cameras focus point in the middle, without always having your subject in the middle of your picture.
As you can see below, I still need to practice some more...
Do you already BBF or is this something that you would try?