Works ok if you have clean undamaged hands, but I work with my hands and they often have nicks, scrapes and burns, let alone the dirt issues. Imagine an inadvertent paper cut and you are locked out of your computer until it heals properly.
The hardware/software used in the iPhone for fingerprints works surprisingly well in my experience. But, as Magic Static points out, any damage on your fingerprint and you are out of luck. I have a few scrapes on the finger I had been using and it hasn't worked for a couple weeks now. Had to switch to my thumb. Fortunately entering the four digit code still works as a backup.
Yes they can be copied, but only if you have access to the person. If your laptop is stolen they will not be able to log it in. It is not meant to keep spies at bay, but keep company confidential data confidential if the laptop is lost.