The SoundClip is just a piece of plastic, and you could duplicate the effect by cupping your hand. But those points alone don't necessarily make it a bad idea.
Priced at $8, the SoundClip snaps on the bottom of your iPhone around the speaker. Tiny molded plastic creates a pocket to amplify (but probably mostly redirect) sound, promising a 10dB increase in volume between 6kH and 20kHz. And when not in use, the SoundClip can actually attach to the phone's charging cord, which isn't a bad idea at all.