It could be good...

It often takes long enough to detect my phone and get ready to wait for the knock to unlock that I may as well have just typed my password in. I type fast enough. I also find myself knocking on my phone, but not at the rate the app seems to like, meaning it takes several knocks before it unlocks. (Maybe I just have freaky fingers, but I also find rapping on my phone with my knuckles kind of uncomfortable.) When it works well it works really well, and is pretty cool.

funkaoshi about Knock – unlock your Mac without a password using your iPhone and Apple Watch, v1.0.2