Let us not forget that a person who has such a device should train to rack once before advancing to meet the enemy. This training should be done so often that it becomes 'muscle memory' and is a reflex upon being awoken to investigate an intrusion.
This device + proper and constant training should make the firearm immediately useable and safe.
I wouldn't own one, but I know many people who would.
But what I've been harping on is this: What happens when one forgets their training under stress? Forget to rack the slide on an empty chamber and the gun is still functional. Forget with this thing in there and the gun is turned into a blunt instrument.