Review (5/5): Amazing!  It works and my teeth are not sensitive!
This product is amazing! After using it for one evening (you do 4 cycles each time you use it) I could see a substantial difference. My teeth look as white as they did years ago. I am a coffee drinker and enjoy fruit such as cherries... both of which can stain your teeth significantly. My teeth are not at all sensitive from this product.
Make sure you use the protective lip care before each treatment cycle. You have to use it on your lips and inside your lips. I think it is because of the gel you apply to your teeth. I didn't get it on the inside for a cycle and the inside of my lip was a bit sensitive for a few hours. But I also have extremely sensitive skin. It felt fine later that day.
As far as the fit of the "universal" tray. For me, it's not a great fit, but I think that is because I had 4 adult teeth pulled when I was a kid (my mouth was too small and my orthodontist had not started using palatal expanders). So I need to hold the tray in with one hand... which is a bit tedious, but totally worth it.
I will definitely continue using this product.