Both me and my wife tried them a little over a month ago and had mixed results. Neither of us smoke or drink coffee, but there was a bit of yellow in our teeth. She had a lot more than me.
The program is you do your top teeth for the first 2 weeks, then the bottom teeth for the next 2 weeks. After about a week I saw a noticeable improvement on my wifes teeth but not really any on mine. After finishing the first 2 weeks, my wifes looked a lot better (comparing her top teeth to her bottom teeth). Mine I could barely tell a difference - I'd say it whitened my slightly, but not really $40 worth. It almost seems like the yellowish tint on mine is actually inside the tooth, not on the outside. I never bothered with the bottom set of teeth on me, but my wife did. Hers aren't pearly white, but definitely better than before.
I did have a bit of discomfort towards the end of the two weeks. They warn you about it on the box though. My teeth got really sensitive and it felt like a slight throbbing pain anytime I'd put the strips on. They say to take a break for a day if it happens, and by the next day the discomfort indeed was gone. My wife had no pain on her top teeth, but her one bottom tooth hurt a lot using them, she might have a cavity or something there though.
So for me, it was kind of a waste of money. For her it was worth it. I'd say if you teeth really aren't that stained, you won't see a whole lot of improvement.