While the other replies make a good point, the app does offer some decent features. I've been using Prey for a couple of years. Now, nothing has been stolen (knock on wood) so I don't have a lot of first hand practice, but as of the recent update it offers a pretty full set of find-my-phone (or PC or laptop) features. If your phone goes missing you can flag it as such either by logging into your Prey account online or by sending a special text message to your phone. Depending on your settings, your missing phone will then send back periodic reports to your Prey web account containing:
GPS/WiFi location information
Current IP addresses
Camera snapshots from the phone camera
You can also remotely trigger an alarm or pop up warning on the phone, or remotely lock it. There are other handy features that make it more difficult for a thief to identify, turn off, or uninstall the app. Check out their website,
preyproject.com, particularly the support page. Great info there. One last note, I
really recommend that you test it out. Flag your phone as missing when it isn't and make sure the features that you need are working correctly. It'll do you know good if something is wrong with the app or service when a thief nabs your phone.
Edit: If you ever lose your phone
in your own home, Prey's alarm feature can be a life saver!