There has been extensive research in the development of the Taser, and in my opinion it is by far one of the best tools police have in dealing with aggressive behavior. That said, I can only relate what my experience has been and what I have read.
We often see Hollywood using the Taser or Stun Gun in various movie situations. Often, we see a Taser deployed against someone and the person goes to the ground unconscious. Even though this scenario makes it easy for a movie director to complete a particular scene, in reality it does not happen. The only way someone will end of unconscious from a Taser is if they happen to fall and strike their head on some object causing the black out.
I have had the Taser activated on me at least three times during training simulations. The effects of the Taser only last as long as the trigger is depressed. The total length of time is 5 seconds. It shuts down immediately upon completion of the 5 second cycle. You have to depress the trigger a second time to start a new 5 second cycle.
The effect of the Taser is the total and complete disruption of the electrical/muscular system of the entire body. This causes the body to tense up and the person experiences a pulsating sensation to the muscle system. Once the cycle completes the sensation immediately stops. The only sensation that I had after being hit with the Taser is similar to a weight lifting work out. All of the large muscle groups feel pumped up.
The Taser is by far the most useful and effective tool we have to use against aggressive or non-compliant behavior. For most departments, its use is authorized at the beginning of the Use of Force Continuum and it is classified as a non-lethal option.
In my experience of deploying a Taser against someone, most do not want a second round. Those who choose to continue in the aggressive behavior and get a second dose rarely need a third. The last person I witnessed getting "Tased" was a real hard case who was wanted for rape and several other felony offenses. He went down and cried like a baby after being hit with the Taser. It only took one cycle to get his attention and compliance. He was taken into custody without anyone being hurt.