Well, at first I tried a box that I had been packing around for a while. After having the first strips in my mouth I noticed it was 2 years past the expiration date on the bottom of the box. I think it was just the peroxide that broke down after the date.
I got a new box of the 14day Crest strips for $19 a grocery, used them all and noticed a pretty remarkable difference. I did not have discolored teeth before I tried them imho. If you have the time I would get the cheaper 14day set from Crest. Less chance of sensitivity problems and they work just as well.
I looked at a store brand, cheaper but it had tons of other chemicals other than the peroxide in the Crest strips.