Do teeth whiteners actually work?

I've been told that over-the-counter teeth whiteners don't work. Something about the whole thing creeps me out. How good can it actually be to bleach your teeth!? Do those things contain bleach? Should I be worried?

I used some sort of whitening strip for 10 days and the results were amazing. I will warn you that there are times when your teeth actually hurt while you have the strips on. The pain goes away pretty fast. The instructions say not to swallow any of the goo that's left behind and to rinse your mouth. It's also best to brush your teeth before using the strips. Trust me...they work!