100%? Hardly. I wish. It's more like 7/10 times in my experience. It can also sometimes be slow.
Also, beyond just the finger scanning, the idea of Touch ID is to eliminate needing to punch in passwords or passcodes, right? Yet in apps like the App Store even after you've set up touch id it'll sometimes ask you "to continue using Touch ID with the App Store please input your Apple ID password". Uh, what the heck? That totally defeats the purpose. It feels half baked. Like Apple is holding back or doesnt trust its users to rely entirely on Touch ID. Quite frustrating. One of the things I hate about iOS and the App Store is the need to input your password every single time you place an order. I thought Touch ID would eliminate that but not really.
And in other apps that use it, like Dropbox, Touch ID is very very slow.
Make no mistake I think Apple has the best finger print scanner on any device and for the most part I like it on my iPhone 6. But it can definitely use some improving. And certainly not 100% reliable. Hopefully one day as they improve it.