I tried it on one of my sites and I love it. It's not just for faces -- any kind of image where the subject is off-center (and if you're using something like the rule-of-thirds when you frame your shot, isn't it always off-center?) would benefit from adding an appropriate hot spot.

The face recognition bit is still sketchy. I found that maybe one out of every ten images, all with obvious faces, were detected. But it's no big deal to select a hot spot -- and as sanchothefat indicates, if you're concerned with good cropping you are going to want to manually select the subject's chin, or nose, or left eye, or whatever manually, to get it just right.