Quite frankly, they have always given me the heebie-jeebies.

I used to work in a research lab that used biometrics on their computer-room access (in this case, measurement of hand configuration, finger length, etc).

I’ve see others that use retinal scans, fingerprints and the like. But my fear has always been that some terrorist who wants to beat the system and doesn’t care about who he hurts to do it will simply amputate the requisite body part(s) – indeed, I seem to recall this being done on Alias, and maybe on "24".

If this is your intended field, I’d like to see you come up with something that will ensure the body parts are still attached to a living person, and that they are not being forced to comply – maybe a scan combined with voice stress analysis or blood pressure?