I had Zoom Teeth Whitening about 18 months ago, it cost about �299 and was the most unpleasant experience.
I had to sit in a chair, whilst some gel was put onto my teeth, then a guard was put into my mouth to keep my upper and lower lip away from the teeth. The gel did cause a little burning to under my top lip. Then an ultra violet light was shone directly at the teeth, I had to sit there, keeping my head still for an hour.
After, the guard was removed and the whitening gel rinsed off, my teeth felt ultra sensitive, I couldn't eat anything or drink anything, except through a straw for about 24 hours and had to take Nurofen for the pain.
As for the results, my teeth weren't particularly stained in the first place, but I did expect much better results. I didn't really feel the whitening treatment made any great difference.