Earlier this year Apple released a new iPhone, the 5S. Aside from iOS 7, 64 bit processor, and better camera Apple added a new feature designed to improve security, the fingerprint scanner. I purchased this phone on launch weekend to replace my aging Android (it still had a sliding keyboard).
The goal of the fingerprint scanner is to reduce the annoyance of passcode entry while still providing some level of security to the device. In order to meet this goal it has to reliably detect my fingerprints and reject everyone else's. While the fingerprint scanner uses a high resolution of 500 ppi, matching is a statistical process and there will be false positives (someone else's fingerprint is valid) and false negatives (my prints don't match). False positives are detrimental to security since they grant access to the device to unauthorized users. False negatives annoy users by requiring another attempt or passcode entry (which the entire fingerprint process was trying to avoid). If there are enough false negatives users will disable the feature.
The use of fingerprints has many unintended consequences. You leave thousands of fingerprints around every day. So while fingerprints may prevent your friends from tweeting "poop" while you aren't looking it reduces the overall security of a passcode by providing a readily available method for sufficiently motivated people to bypass the passcode. Fingerprints also have legal implications. Unlike passwords, fingerprints are not protected under the fifth amendment. If either of these reasons bother you don't use the fingerprint scanner.
Now that we have the theoretical discussion out of the way let's discuss my experience.
Adding fingerprints was simple: just keep tapping the home button until the on-screen fingerprint's ridges turn completely red.
For the first week the fingerprint scanner was fantastic. Only once was I told to retry and even then it was due to user error (I had attempted to use an unregistered finger). As I became more comfortable with the phone I began to grab the phone in increasingly strange orientations. Turns out that the fingerprint scanner is useless unless your finger is within an extremely small deviation of the angle used for saving the print initially. This caused much frustration until I characterized the problem, at which point I promptly removed my fingerprints. The fingerprint scanner is a noble idea but the lack of flexibility undermines potential.
The fingerprint scanner does however bring one feature which all users will appreciate: a sturdier home button.