Have been using it since Wednesday (3 days now) and I think it only failed on me like twice, but it didn't fail completely... I just adjusted my finger position and it then worked.
I registered my thumb... and I registered it the way I use the phone. In other words, when I added the print, I held the phone the way I will normally hold the phone. I didn't register it as if I was getting finger-printed at the police station. LOL
I also registered my index finger because sometime when the phone is sitting next to me, I'll reach over with one finger just to check it. In that case, I also registered the print in the way I will touch the button.
I'd suggest that when you register your finger prints, you register them in the way you will use them... hold the phone the way you normally do.
If you've done that already, I don't know what to tell you. It's working nearly 99% of the time for me.