Self Image uses sophisticated face-detection algorithms to let the iPhone camera know when it has a face or two in the field of view, at which time it begins a countdown to get you ready for a photo. The app is simple to use -- you launch it, press a button marked with a "1" if you're taking a photo of yourself, "2+" if taking a photo of yourself with a friend or two, or "self-timer" if you want to just take a photo with a self-timer.
If you're taking a self-portrait, Self Image searches for a smiling face (or faces, if you are with friends), and then begins a three-beep countdown when it finds one. When the picture has been taken, it is saved to your camera roll. In practice, this works very well provided your face is properly lit. In dark conditions, Self Image might have trouble finding your face, at which point you'll want to find some light.
Self Image is definitely tuned for human faces. I tried unsuccessfully to get it to recognize my cat's face, and I only succeeded in annoying the cat. Be sure to take a look at the gallery below for some screen shots of the app in action, although unfortunately the only portraits you're going to see are of my face. Arrrrgh.