I avoid anything with biometric sensors today. They tend to sound good on paper, but when it comes time to use them they often leave a lot to be desired. For instance, my Dell notebook has a fingerprint scanner that seldom actually lets me into the computer.
Apple has filed a patent application that shows an interesting way the firm is looking at telling if the user holding a future iPhone is authorized to use it. The app shows an iPhone with sensors in the case that reads the users heart rhythm.
The heart rhythm is somehow analyzed to identify the person holding the phone and allow them access. This is a cool idea that would prevent unauthorized access to your iPhone and keep you from having to do anything but pick your device up.