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The apple version works very well, if your device has an internet connection. So if it is an iPhone, no problem. It will work every time so long as locater service is enabled. If it is an iPad, you can find it if someone connects to an active WiFi, or if it has the cellular service, and the cell service is enabled. When I first got my kids an iPhone, within a week, it was stolen at the park. I used the Find My iPhone app to locate it, then I knocked on the door of the house it led to and the father tried to tell me it wasn't there. I sent a tone to it, and it was in his kids pocket. I got it back. Great job Steve Jobs.
You might have read about someone breaking into the late Steve Job's house this month and stealing a bunch of stuff. It turns out that when you use a stolen iPad that belonged to Steve Jobs, Apple notices. They notified the police, a search warrant was issued, and the burglar was arrested. So if your name is Steve Jobs, I'd say the chances are actually pretty good that your gear can be recovered. Whether the police would actually go the the trouble of getting a search warrant if your name isn't Steve Jobs is a different question, judging by the reluctance of my local police to even bother taking a police report when my garage was broken into and my lawnmower stolen.
As far as Android goes, probably the best know app for locating lost devices it Lookout, but there are others as well. I have AVG antivirus on my Android, mostly because it has a free locator feature. You can log on via computer and turn on GPS remotely. I actually tried it just a few minutes ago to see how accurate it was, and it placed my phone in the middle lane of the 4 lane highway going by my office. I think that reflects a poor GPS signal more than a limitation with the app, but if it had actually been lost or stolen I would possibly face the same issue. You can also remotely activate the ringer to give audible guidance when you have narrowed it down.
I think perhaps the most valuable feature of these apps is the ability to remotely lock and wipe the device. From my admitedly casual testing, I would probably not have been able to specifically locate my device, but at least I would have been able to erase any private data so that the damage would have been limited.