I have had the s5 for around 4-6 weeks (unsure). I've tried to use this several times and it's about impossible to get consistancy.
I will admit a few things however to be fair (because I want it to be flawless also).
1) I do not know how to use a horizontal swipe setup, but the vertical is very difficult [for me].
2) The vertical is no problem with over 90% 1st swipe success for my sister (5'2, small hands)
3) I am large, 6'3 270 with big fingers. I found the index is much easier to use than thumb [for accuracy] but often bounce from 80% 1/2nd swipe to nothing.
I had originally hoped, similar to the heart mointor thing, you could simply hold your finger somewhere but I guess that's a bit too much to ask for (real biometrics). I am new to smartphones and I'm amazed at what this thing can do so I guess I thought it might do that too lol
I don't lock my phone because passwords are annoying to enter a lot (low timeout for battery) and pins are way too easy to guess with time if someone wanted in. I want it to work, and when it does it's really cool and quick. I will have to investigate this horizontal method but also hope they keep improving it.
I believe it's merely my large[r] hand/fingers that are the issue. I found a few offsets that brought on a 7/10ish < 4 swipe success but this is time consuming and in times of quick need to reply to someone in text (say your lady) it can be frustrating when it don't work.
I would not call it straight up crap, but I do think it can be tricky using vertical if you have larger hands and fingers. I will keep messing with it here and there because I love the idea.