Here is whitening broken down in a nut shell for you. Essentially first you have to look at the type of stain that it is. Many stains are extrinsic in nature (i.e. food, smoking, beverage stains) these can with limited success be treated to some extent with over the counter whitening tooth pastes. But after the extrinsic staining is gone, you're left with the natural color of your tooth. Intrinsic stains are those that make up the natural color of your teeth as well as those caused by various genetic conditions and/or antibiotic treatments during tooth formation/ maturation. You can use as much whitening tooth paste as you want and it won't touch these intrinsic stains. This is where whitening gels (essentially carbamide peroxide and to a limited extent hydrogen peroxide) come into play. To sum these gels up, yes they all work, certain stains and underlying tooth colors are more susceptable to the effects of whitening gels. How they work is simply a matter of time and gel concentration. A low strength product such as crest whitestrips with 7.5% carbamide peroxide as I remember will take twice as long to achieve the same result as the standard office delivered gels which are in the 15% range. In my office, we will commonly jump start the bleaching process with a one hour treatment of 30% carbabmide peroxide followed by sending my patients home with syringes of 18% gel. Some of my sensitive patients get treated with 5% gel(generally Colgate Platinum extra sensitive formula) over a longer period of time.
The OTC products you mentioned do work. I've found in my practice that the whitestrips are quite successful and popular, although my patients like the results on their maxillary arches(upper) than the mandibular arches since the strips are easier to conform to the upper arch. Some patients will have a small area at the gingival margin where the whitestrip doesn't reach 🙁 The with the home formed trays, you need to be careful in their contour at the gingival margins, extened bleaching gel exposure to the attached gingiva at the margins can lead to gum irritation and potentially gum recession.
Bleach away, it works, but you need to have some patience with the lower concentration gels. 😀