I went to have my teeth whitened after buying a voucher with Groupon for £44, usually would be £150. I was a bit sceptical as it was so cheap.
I arrived and was taken into a beauty therapy room where we measured the colour of my teeth against a chart. The chart went from S4 to S30, or something like that, with S30 being the most yellow. I was measured as an S20. I wouldn't say I had horribly yellow teeth or anything, just normal really. I just wanted them to be bright white and healthy looking.
The assistant, who was lovely, then placed a plastic contraption in my mouth to show all my teeth (I looked like Wallace from Wallace and Gromit).

It was quite comfortable at first. A blue gel was applied to my gums to protect them and then another gel was applied to all of my teeth. Then a lamp with a UV bulb was placed over my mouth and the assistant left the room and turned the lights out for 20 minutes. It was okay at first, a bit odd. I could have done with a friend there to entertain me really as it was a bit boring. It did become a bit uncomfortable towards the end. I was grateful for the relief when she returned after 20 minutes and let me swill my mouth out. She then applied a fresh layer of gel and I went through the process again for another 20 minutes.

Then after that I went through it again for a final 20 minutes. So 1 hour in total under the UV bulb. My teeth did sting a little bit in places, but nothing unbearable. More like a really sensitive feeling.

I swilled my mouth out for a final time and peeled off the blue gel from my gums. When I looked in the mirror I was really surprised. The difference was noticable straight away. My teeth were so much whiter than I thought they would be.
We measured the colour again and this time I was an S8.

I was really pleased with the results. I could only eat and drink white coloured foods for the next two days, which wasn't as bad as it sounds. Any excuse to eat a bar of Milkybar! And we could all do with cutting down on tea and coffee. I didn't experience any more sensitivity. I have a small calcium spot on the front of one of my teeth and this was highlighted immediately after the process. The assistant assured me it would settle down and she was right, it's hardly noticable now.

Funnily enough the following morning on 'This Morning' they were reviewing different teeth whitening methods and apparently some places charge up to £600 for this process. It was also confirmed on the show that this is a safe process and should last about a year - even longer if you don't eat too many staining foods (coffee, curry, red wine etc).
Would definitely recommend, and now I have had it done I think it would even be worth paying the full price at £150.