My dentist told me going in that my results would be subtle at best because my teeth are "gray" in color. The best candidates for Zoom have yellow or brown teeth.
I had a fluoride treatment beforehand and used a special toothpaste for six weeks before the treatment. The treatment itself wasn't painful, it took a while to finish but they had movie goggles to pass the time. That day I had bad "zings!" or twinges of pain through my teeth, but when I say bad, know that I am a big baby when it comes to pain.
My dentist was right in that my teeth were only a few shades lighter, the effect was disappointing, but she warned me up front and I did it anyway so I don't blame her or the procedure. I wanted to try it, I tried it, I personally wouldn't do it again.
I didn't bother with the bleach trays afterwards because the twinges weren't worth it for the minimal improvement I'd see. But I've seen people she did it on who started out with yellow or brown teeth and wow! Huge difference. They now have million dollar smiles.
I promised my husband that when he quits smoking I'll pay for him to have it; unfortunately it hasn't been enough of an incentive, LOL.