It's a really good idea, but seems to need the iPhone to be unlocked in order to appear on another device, therefore no request notification is even possible such as when a Facebook notification appears on the lock screen, and indeed no way to send (or request to send) to a locked device. Perhaps as this is because the chips are off when device locked? Either way a disappointing aspect of a good communications feature!
the device doesn't have to be unlocked, the screen just has to be woken.
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Never used it. I just send stuff as text and email.
try it, you will love it.