We have one in a pin protected key safe. Put it up after toddler DS2 shut the door on me when I went into the front garden to put the rubbish out one day. Luckily his older brother was able to reach the lock and let me back in.
It's incredibly handy, means other people can get in if they need to without having another key cut, and that I can leave the house in a hurry without having to look everywhere for my missing keys first.