I use the crest white strips. I love them! I kind of mash the stip on my teeth, making sure to get the goo inbetween so the top isn't white and stained on the sides, make sense? Also, talk about freshening up your mouth! Woo Hoo!
If you have a crown or any bonding, be careful of whitening too much and creating a mis-match of color.
I see a color difference in 3 days and I stop there. It's different for each person. My husband isn't wild about the shape of the strips fyi. The bottom and top stips do not cover every tooth. Just the front 8 or so.
Good luck!