I paid �300 to have mine done 2 years ago. I had Zoom Whitening and it wasn't a nice experience, my teeth were hyper sensitive for 48 hours and I had to take pain killers.
My teeth were never overtly stained or yellow, so the results weren't as dramatic as I'd hoped, so it probably worked better for those who smoke, drink red wine, coffee or tea.
It's relatively safe and one treatment is enough for most people, but having it done too much will probably cause the tooth enamel to erode or become porous, causing sensitivity.