A few years ago, I had whitening trays made through my dentist. They cost about $150, and they were fantastic! After a year or two of use, they were somehow lost, and since then I've tried various strips and trays for whitening and haven't found anything that works even half as well. Even standard boil-and-bite trays are useless, because they're so thick that they don't retain the peroxide gel properly and it's difficult to breathe when using them. These thinner trays are great so far, though -- definitely a very close second to my professionally made trays, and for the price, there's no question that these are the better option. They don't fit quite as closely as my professionally made trays, but they're snug enough to stay in place without the frequent readjustments required by other products, and they keep the gel in. It's even possible to talk with the trays in (though not very well). These are definitely the best of the many DIY whitening products I've tried.
Advice: The directions say to hold the trays in boiling water for three to four seconds, but I found that two second was about the longest that I could put them in the water without completely deforming them. I had to go through all six trays before I got an upper and lower with which I was satisfied. Make sure to slide the trays in so that your molars don't fold the backs of the trays inward -- lumpiness in the back causes the front of the trays to fit improperly (although depending on what it looks like, you can probably trim the backs of the trays to fit better).
Overall, the best home whitening product I've tried so far!