Let's hope you never need Where's My Android. The sole reason for this Android app's existence? You've lost your phone or it's been stolen. Neither case makes me feel warm and fuzzy, but something tells me these situations are common. My testing shows the app works, and therefore I feel it's worth having around, just in case.
It works via text message. You send your lost phone a message with a specific "attention" word or phrase. If your phone is on, but silenced, Where's My Android will turn up the volume and ring for either 30 seconds, one minute or five minutes, whichever you specify. This mode works best for phones lost in your home, car, office, etc.
There's also a GPS mode for phones that are really gone, baby, gone. Text the special GPS attention phrase to your Android, and it will text back a Google map link showing its location. Think of it as a helpful "wish you were here" message from phones that have left town.
Overall, it feels easy, but if you have any trouble, a quick tap from within the app takes you to a video how-to on YouTube. Keep in mind, in order to work, Where's My Android must run in the background all the time. If you Force Quit the app, your lost phone will stay lost.
I tried both beeping and GPS modes, and Where's My Android works as designed. For me, it's worth running it all the time. I didn't notice any significant battery drain.