Zoom will dramatically whiten your teeth, but be prepared for the pain. I read these reviews about the "zingers" - well it feels more like biting down on aluminum foil or having small waves of electricity shock your teeth. It started to really hurt towards the end and was almost unbearable. My teeth however were 5 shades whiter.
I'm glad my teeth are whiter and that it's over with - but if you are sensitive or do not have a high tolerance to pain, think twice.