Both my girlfriend and I had this done sometime in 2010 at Dr. Apple Dentistry, Cebu, Philippines (Ayala Mall). We had separate dentists perform the Zoom procedure. I was 30 at the time, and she was 26.
During the two hour treatment, there was minor discomfort with the long stretches of keeping our mouths open. After the treatment was finished, our teeth, gums, and lips looked fairly bad for several hours so we couldn't go out anywhere. The minor discomfort continued on after the treatment and then several hours later, the pain increased. I would say on a scale of 1-10, the pain was roughly four.
I used the provided tray that night and then the pain raised up to about a five, which in my opinion is still manageable, but annoying. I had a very difficult time sleeping that night because the manageable pain had started to turn into a throbbing headache. This headache continued on for about five days and made me completely miserable!!!
My gf's experience on the other hand was considerably less. She too was in pain and had a horrible headache, but hers lasted only about three days and the pain level was probably about a three.
As far as the whitening goes, my teeth (which were whiter to begin with) raised up about five shades. While her's had a more drastic increase of about nine shades. We were both extremely happy with the whitening once the pain went away and we realized the results. I would say they stayed pretty white until maybe six months later.
It's been about two years since the procedure and we are beginning to look around at whitening our teeth again, but to be honest with you, I'm a little scared now. I can't help but think about the pain from last time and it just makes me cringe.