I use the back button autofocus all the time, specifically for action and wildlife photography. But if you have the choice you don't want to use the * (asterisk) button, you want to use a dedicated AF-On button instead. The * button should be left for locking the exposure so that you can recompose the scene wherever you want after first locking the exposure. I don't actually use this feature very often, but I appreciate that it's there. Most newer cameras have both a * button and an AF-On button. But I digress...

Separating the Focus System from the Exposure Metering and, more importantly, the shutter action, is fantastic. I can now set my focus once and take as many pictures as I want because pressing the shutter button no longer changes my focus. It still meters correctly, but it doesn't refocus.

I love having independent control of the focus and the shutter button. If my subject is moving then I can choose to refocus. If not I can immediately shut it off (or rather not turn it on).

Basically, you get the best of both worlds between One-shot mode and AI-servo mode. You can leave your camera set to AI-servo mode, but if you want the functionality of One-shot then you just take your thumb off the AF-On button. It's great!

This is kind of a strange concept until you try it. But if you really try it for a few days I would be surprised to hear that you didn't love it!

By the way, I shoot Canon, but as I understand it the setup of the AF-On button and metering via the shutter button is virtually identical between Canons and Nikons.

I can see real value in the separation, but I think Canon did it backwards. Unless the light is rapidly changing, the metering is likely done once, but focusing may be done many times before taking the shot, especially with moving subjects. In that sense, I suspect that it isn't that great for action photography in the current setup. Maybe that's why so many sports photographers shoot Nikon... ;)

I think Pentax added one button to their camera line that is a fundementally superior design choice and that's the "green" button. In manual mode, for example, I press this button and the camera meters the scene and sets an initial aperture and shutter speed which I can then quickly adjust with the front and rear dials for the desired depth of field or effect. After that, I compose, focus, and shoot knowing what I've defined. It's very, very, handy and I'd absolutely miss it if I was to switch from Pentax. Heck, even for manual lenses it's handy because you can use it to do stop-down metering and adjust the lens based on the result. A function that has allowed me to buy some truly excellent optics at bargain prices!

So, in a nutshell, a good concept from Canon, execution seems iffy.