I got my teeth cleaned last weekend and my dentist gave me a "teeth whitening trial" for free. She applied it on my teeth for about less than 5 minutes, then removed it.
When I rinsed my mouth, I can taste acid... which I didn't like much. But other than that, my teeth did look whiter (but not in-your-face-white) immediately and my teeth did not feel sensitive or pain at all. I think I will give it another try next time and pay for a full session.