I'd say it works about half of the time. Set it up for both of my thumbs and every time I press the home button it seems to unlock my phone about half the time. It's literally 50/50 'try again' and successful unlocking. I've tried re-doing the setup for it about 10 times now, doing exactly what it says on the screen and I just can't make it work properly. My hands are not greasy, my 5s is perfectly clean and still it doesn't really 'just work' the way you'd expect it to. You seem to have to do it at the right angle every time if you want it to actually work properly, align your finger 20* to the left or right and it'll just say 'try again'.