My understanding is that even "success" is considered like a 25% accident rate, there is a lot of laundry and rug cleaner involved. I admit that I haven't looked into it closely, but it seems to take a huge time and effort committment for a very small payoff.

On the other hand, I've noticed that (because it's in their own best interest) a lot of diaper companies are trying to up the age of potty training (for example, "Pull-Ups" advertises their product with lots of older kids using their diapers to show that "I'm a big kid now.")