It's very useful for bird and nature photography.
1. I use the telephoto as instead of a telescope. So, I can focus with the back button without wasting battery on IS.
2. I use A1 servo for focussing. There are a couple of situations where it is useful to uncouple the focussing. Sometimes it is difficult to lock on to a small target because the AF darts backwards and forwards from the subject to the background. You can lock on to the target and then release the back button and them take the photo. Or you can sometimes want to focus on a branch on which the bird is standing and then hold the focus as you recompose. Similarly, if you think the bird is going to take off sideways, you can focus on the static position and not worry about losing focus as it leaves its perch.
I generally feel more in control being able to focus independently and I know it has increased my keeper rate.
I always use back-button AF. It's great in many One Shot situations. Where the distance-to-subject is not changing I don't have to wait for focus, I just snap. If I've recomposed slightly after focusing, I can shoot many shots without moving the camera. It's even better in AI Servo, where I can independently control focusing and the shutter.
Here, here! I always use the back button focusing option. I thought it was silly but Artie Morris recommended it to me and encouraged me to try it. Just for giggles, I did try it... I didn't like it for 2 days but promised to try it for a week. I was SOLD half way through day 3! I've never looked back and when I pick up someone else's camera I find it awkward to use the shutter button for focusing instead of metering and shutter only... drives me nut!
I use it for portraiture, wildlife, BIF, landscapes, the whole gamut! Give it another day or two and feel the power of more control in your fingertips!
All the best!