Well.. A determined hacker or a so called BAD man can find a way to break into anything. Biometric devices are not completely immune to attack. As customers and the industry learn to what length intruders are willing to go to penetrate these new defenses, we will also continue to find the means to thwart their attempts. The point is to balance the solution to the threat.
It is the task of biometric products to apply a mathematical algorithm to the identifier (retina, or fingerprint for example) to determine if a user is a who they are pretending to be. In devices where a finger print is the means of identification, a small device will scan the fingerprint of the user and momentarily record the fine details. Where fingerprints are concern this may include features such as the finger prints' hills and valleys, the direction and branching points, line endings and dozens of other minutiae.
Restricted access is good, but what if you are rejected during the authentication procedure from accessing your own computer? It can happen if you have a large cut on your finger for instance. In such situations you would be in the same position as if you had forgotten your password. Heh. What would happen if you were just rejected for no reason, or even worse some one else had a finger print similar to yours and was able to gain access your computer? Well the odds are statistically in your favor that neither of these situations will occur.
All in all biometrics is the final piece in the security triangle and the most secure.