I read a lot of these reviews and I think the problem for most people is their dentist's lack of experience about the procedure. I had moderately yellow teeth beforehand, and drank a lot of coffee, which I think made me a better candidate for Zoom than people who have mild tints on their teeth (in which case you wouldn't notice much of a difference afterwards). I've also had sensitive teeth my whole life (mainly to cold drinks and foods), and although I experienced some MILD sensitivity during Zoom, it was nothing more than I would already experience by eating ice cream, and the sensitivity diminished soon after (by the next day I was fine).
Really, just make sure you know and trust your dentist and dental staff--you can usually tell when doctors truly care about their patients and when they don't (and I've had plenty of experience with doctors who don't).
I definitely wouldn't recommend this if you only have mild yellowing because of the expenses. Those people are much better off doing the tray whitening system (which is what I now do for any necessary "touch-ups"), which is cheaper and will probably be just as effective for you (since it's mild).
I had Zoom done almost 2 years ago and I noticed an immediate difference that I absolutely loved. I've done one touch-up since then, not because the stain returned to how it was originally, but because I was going to be in a friend's wedding and wanted my smile to look its best. Honestly, as long as you're willing to find a good dentist (which may be pricier than others), and you have more than mild yellow staining on your teeth, I would hands-down recommend Zoom. It's the easiest thing in the world to sit in a chair for an hour and a half and suddently have a white smile. During the procedure I was able to listen to music or watch TV (they have TVs in the exam rooms for the patients at my dentist).
So final thoughts: do your research on your dentist and you should be good to go. Good luck!