I haven't tried any whitening system, because I have always been scared of damaging or debilitating my teeth. I don't know why everyone is so obssessed with white teeth, anyway. I don't think teeth were intended to be pure white. Besides, I don't think anything topical, unless it is penetrating (like in proffesional treatments)is going to remove yellowness, where yellowness is something that comes from WITHIN the teeth. For that reason, I fear that the outer "layers" of the teeth will be debilitated before the solution reaches the inside of the teeth.
When I go to the dentist for a cleaning, I come out with transparent teeth that are yellowish inside. For some reason they look more yellow (inside) after a professional cleaning.
I wouldn't risk the health of my teeth just to sucumb to the Pure-white teeth obssession. I maintain proper higiene and I think that should be enough. Also, certain foods tend to stain, like dark souces, or rub your teeth from the outer layer, like sodas. These are foods that are not clean, anyway. So I avoid them.