My dentist made trays from molds of my teeth so they fit perfectly. He then sells the gel that is 10%, 15% or 22% whitening chemical (hydrogen peroxide I believe). I use the 15% and only put a dot for each tooth in the front of the tray, you then press them on and wear them the first time for 4 hours after brushing and flossing. After that I wore them again for about 2 hours one week later. Now it has been months and I may wear them again some time soon, since your teeth will become darker again. I like the perfect fit, the % gel that was recommended (my husband used the 22% but my gums are too sensitive for stronger whitening), my dentist will check that all fits well if I want, and I can re-use them until the rubbery molds get old and start cracking (no signs yet after a little over a year and basically 2 or 3 uses). The cost was $500 total including 2 tubes of gel.
I would do it again, the molds that you heat and then press onto your teeth are not nearly as close a fit, the Crest WhiteStrips tasted awful and did not make my teeth whiter at all, and every kind of toothpaste seems the same to me (but I have not tried the 3D). A girl at work who is a lot younger than I am uses the Whitestrips frequently and has amazingly white teeth, so perhaps you have to keep using them, but I just did not like the taste at all.