I have worked in the dental field for almost 6 years.
Whitening has been used for some time and results are becoming more and more satisfactory. Side effects are minimal and usually short-term (such as increased sensitivity.)
The take home OTC products contain a small percentage of peroxide. The reason they cannot be stronger is that the solution in stronger concentrations can damage gum tissue and the OTC products are sloppy to apply.
So, if you want more dramatic results (10 shades whiter or more) go to your dentist and get either the take-home kit or do the chairside. The prices quoted above are fairly typical: $250 to $300 for take-home kits, $500 to $600 for chairside. The advantage of both is that they are custom designed for YOUR mouth so the solution stays only on your teeth and does not damage your gums. The chairside has the advantage of being "instant", usually two hours total time and results are usually more dramatic. If you are faithful about wearing the take-home trays for up to 4 hours per day, it takes about two weeks to get results.
Watch for cosmetic dentists running ads for free bleaching for new patients. Often, cosmetic dentists use the bleach kits as a give-away to attract new patients. The get you in to whiten, then sell you on veneers or other cosmetic restorations to restore the shape of your teeth.
Either method of bleaching will give MUCH better results than OTC preparations. You will get better results if you have your teeth cleaned by a professional before whitening to remove plaque.
Remember: restorations, such as fillings and caps, will not change color, so if you are going to go with a dramatic result, you might want to replace any restorations in your "smile line."