the first 2 days I had the phone, i didn't yet have a case on it. During that period of time, I would say close to 20%. It was terrible. I mean, I really really hated the phone, to the point I was considering selling it (or returning it and taking the restocking fee hit).
Man, I am glad I did not do that. I wound up buying a cheap, thin case on Amazon (clear Spigen case) and popped it on. Ever since then, I am pretty close to 100% accuracy on the fingerprint sensor. The case (not just this case - any case really) acts as a natural guide for your finger to hit in order to unlock. After a few days, you will get the hang of it easily.