Hi Dajaw,
I recently started using the Crest Whitestrips about a week ago, so I'm only halfway through the box (it takes two weeks).
I bought them on sale at a drugstore, Rite-Aid, for $25.99 US.
I do think it's making a difference already, and am anxious to
see what happens over two weeks. I have heard that improvement really depends on what the nature of your tooth
yellowing might be -- in my case, just years of drinking coffee so
it's pretty minor discoloration.
You can read some other opinions here:
http://www.epinions.com/Crest_Whites...splay_~reviews
Although there is a slight "tingle" at first, it's certainly not
painful and not a big deal as I wear them during my
commute to and from work.
Hope this helps!
Sabrina