Lots of good
reviews on the internet about this but let me make it simple, the pricier the teeth whitening kit, the more peroxide it will contain, the whiter your teeth will get.
Unfortunately (or fortunately for us), these kits doesn't have much peroxide, so they are safe to be used by like 99% of the population but since they doesn't have much peroxide, you will never get super bright white teeth with it like they show on the cover of the kit
To get the best results go see your dentist, he'll be able to recommend the highest peroxide treatment that you can handle for you and he has more tools at his disposals to get the job done.
I don't have the money for the dentist but I tried about 9 months ago a 30$ kit, well it wasn't so bad, they do are a little bit whiter now. They said on the kit, don't use more than once every 18 months but I'll probably use it again this year.