I personally wasn’t impressed with Knock. Not only for the reason that it didn’t work well, but simply because I don’t think typing in a desktop password on a computer is that daunting of a task.
I played with Knock a bit yesterday and came to the same conclusion. Activating it is much like knowing the secret knock to get into a speakeasy. It’s very picky about which knocks it will accept, and they have to be done in close succession. These two requirements led to me beating the hell out of my iPhone. Undesirable.