I bought the Professional Level 4 Crest 3D Whitestrips to get the maximum effect.
I was worried about sensitivity, because I have generally sensitive teeth anyway. But I was pleasantly surprised. After reading many reviews, I decided to use them just once a day, instead of twice as per the instructions, and this worked well.
Just four days later I had significantly whiter teeth, and very little sensitivity.
I was skeptical about this product, but I'm so glad I decided to give them a go. I had thought about gong to a professional to get them whitened but I saved a lot of money this way!