People have said they get severe tooth pain during and after the procedure. I think they called them 'zings'? You can search on the internet and there's tons of posts on the tooth pain. They described it as sharp, shooting pain through your tooth up into your head. Some people had it so bad they missed days of work.
I did LOTS of research before I got my teeth whitened. I have a discolored front tooth (softball hit me in the mouth), and everyone said it would never whiten and I'd need a veneer or a cap. So I wanted to whiten my teeth before I got it done, so all my teeth would match the 'fix' on my tooth.
In my search I learned that Zoom whitening does the same thing the bleaching trays you get from the dentist. The only difference is Zoom is done in 1 day, and trays can take a few weeks to a month (for a few shades lighter - probably more for smokers). After Zoom they send you home with a touch-up kit, too, which is the same thing as the regular bleaching trays you do for weeks. Oh, and the price is MUCH less. Well, after I learned about the tooth pain (there were a lot of complaints I found), and that I'd get the same results, I figured I could wait a few weeks to get the same color for half the price.
I tried Crest and all the other OTC ones in the past, but they all wasted my money. If they did whiten it was just a little, but it never lasted. I finally went to the dentist and got my trays. I use Day White by ACP, which is 20 minutes once or twice a day. I did it for a month, and my teeth practically glow in the dark (well, not quite that bad!), but they are very white. And I got a bonus - it lightened my dead tooth to the point I don't need the cap/veneer after all! I still have whitening gel left over - probably enough for another 2 months. I can use that later for a 'freshening up' every 9 months or so.
I am SO GLAD I bit the money bullet and went to the dentist to get the whitening trays. They are very much worth the money. I would say that anyone who is serious about whitening should do it.