I think that this is very cool, and that people who are reacting negatively to this generally have no idea what they are talking about. Amazon will be doing this in full support of the FAA, with cutting edge technology. There will be layers of redundancy that ensures these can't just come down on someone. It's inherent to the design of quadcopters that they can survive the loss of one engine, and most designs are capable of controlled flight on just two.
People seem to be caught up on the idea of drones invading privacy. The truth is, that the police have been using unmanned machines in the air to spy for years now. This is in use already by police departments all over, so fighting Amazon on this issue will change nothing. And I think that police using drones is a problem, but it is a problem entirely separate from Amazons use of remote controlled quadcopters for delivery and these issues should not be confused.
As for criminals employing drones with built in guns... do you know what happens when you fire a gun? No civilian grade quad copter would be able to compensate for the kickback of firing a projectile; not to mention the fact that is just an absurdly difficult and expensive replacement to the tried and true method of robbing someone; without any real increase in safety since you need to be nearby to control the copter.