Review (1/5): Be careful!!
I hated this stuff. It gets EVERYWHERE. Stains all over my sink and countertop everytime I open it. I'm very careful but even the slightest bit of dust... the black charcoal looks like a spoonful. Pain in the ass.
As far as the effectiveness, well, I'm guessing it helps a little. I never saw a huge change personally, but I do think it helped with whitening my teeth. The degree to which is did is hard to measure though.