Review (5/5): They work!
First, this product really *does* whiten teeth--it's not a gimmick as some people think. (It uses a peroxide, which is probably how your dentist would do it, although this is a less concentrated form and takes two weeks.) Of course, it only whitens natural teeth, not dental work, but for most people, they should work.
Now, on to the product itself. Some people have complained about them being difficult to apply, but after the first couple strips I didn't have any problems, and even the first time I applied it I didn't fold a strip or mess up anything major. Keep in mind that some tooth sensitivity is normal; personally, mine was only mild at worst and really only noticeable right after I applied a strip, but I know some people who aren't as lucky. If this is you, using a toothpaste for sensitive teeth may help. Also keep in mind that eating, drinking, and probably talking (unless you have better luck than I) are pretty much out of the question while you're wearing these, but I simply did one application on my drive home from work and another later in the night. My only concern was that the strips, especially the top ones, are a bit short and only cover your front eight or so teeth, but for most people that shouldn't be a problem.
A year after my first use (teeth still looking good), I am currently using a generic brand--Equate, available from Wal-Mart for about $15, about $5 less than these--and have to say that I like Crest Whitestrips better because the strips are more flexible and easier to apply, and they stay in place better. The Equate ones have a bit harder plastic and tend to slide off my teeth if I open my mouth. Hardly worth it to save $5.
It is also worth noting there are two other versions of Whitestrips: Premium and Premium Plus. Both contain a more concentrated peroxide that works in less time (7 and 10 days, respectively [the 10-day version is the same as the 7-day version but includes three extra days of strips for extra whitening], as opposed to the 14 of this Classic version) but are more expensive and--based on their reviews--seem to cause more problems with sensitivity and such. Therefore, unless you're in a hurry, I think you'd be fine with these.
In summary, I am happy with this product and think that most people would be, too. Not bad for the price (certainly cheaper than a professional whitening), and they really do whiten!