I can notice results from this product almost immediately and the results are definitely more pronounced with prolonged use.
I spoke with my dentist and they say that this is hands-down the best over the counter whitening product on the market. It is worth noting that extensive use can make your enamel more brittle, so that is something to be mindful of.
My main issue with this product (and pretty much any tooth whitening product..) is that it is very irritating if you have sensitive teeth. It is somewhat painful to use and my tooth sensitivity is increased for a day or two following a whitening treatment. This is not an issue for everybody, but if you know you have sensitive teeth, know that this product will not be comfortable to use.