I've had Zoom Light Whitening. If your teeth are naturally yellowish (Most people's teeth are not naturally white) then it's unlikely that your teeth will emerge brilliant white after treatment, but there is usually a substantial improvemnet.
When I had mine done, the technician put a plastic clamp in my mouth, so the front teeth were bared, then the whitening gel is painted onto the teeth. I had to sit in front of an ultra violet light for an hour before the gel was removed. (I paid �300)
Afterwards, my teeth were extremely sensitive and very painful. I could only drink through a straw and even air made the pain worse. I had to take Neurofen and the pain only subsided after 24 hours.
My teeth after treatment looked a little whiter, but then they were never particularly discoloured beforehand. I had mine whitened about 2.5 years ago and I think since then, there have been further advancements on whitening, so you may need to research further.