At a Glance
Metal Detector is an app that falls into the "wow, that's cool" category, but don't expect it to turn your Android device into a replacement for a real metal detector. Several reviewers have commented that this app could more accurately be described as a magnetic field detector. My tests seem to bear out this assertion: Electronic devices and live power cords will trigger Metal Detector; ferrous (iron-based and therefore magnetic) metals will trigger it too--but nonferrous metals like aluminum will not.
Metal Detector is simple to use: Just move your phone around until you come close enough to some metal to trigger the detector. Your phone will vibrate or make sounds (choose via the Menu key) to indicate that metal is present. Metal Detector can also be calibrated or "zeroed" and adjusted for greater or lesser sensitivity to produce better results.
Metal Detector is a great, fun demonstration of the incredible versatility of the Android platform, and what kind of hacks or modifications may come from it will be really interesting to see. At this point it is mostly just an interesting and entertaining novelty, but it is still worth trying just for the fun of it.