I hated those damn things! Laying on my back with cotton mouth so they wouldn't come off during......they slide around, give too much tooth sensetivity for the little whitening you do get.....
Use the natural white kit (more $$ but so worth it) with the trays as it gets front, back and all teeth at once, you only need to use it one time once in a great while and less sensetivity. I use sensodyne and the monthly treatment for sensetivity anyway as I have real sensetive teeth, but hell yah the bleach hurts your teeth. Even the dentist stuff does.