I think 99% of the drugs are useless -- other than the fact that they do exactly what your niece is experiencing: motivate you to eat right and exercise because you're spending so darn much on the drugs that you might as well make it seem like they're working! The same thing happens for some people when they join a gym.. they could have done it without the equipment, but spending the $$$ every month is the kick in the butt that leads them to lose the weight.
I will say that commercial meal plans (mentioned in the poll) do "work" for most people, because it's just another way of controlling your calories. Of course, that's only if you follow them... if you eat a bag of chips alongside your Nutrisystem meals, of course you're not going to lose weight.

I do think that they set people up to either ALWAYS stay on the plan or end up gaining some weight back because they haven't really learned how to prepare and choose the right amount of food. Yeah, you can look at the meals you're given and figure out how to do it on your own, but I think it's a minority of people who really DO that. So do they lead to weight loss? Sure. Do they teach you to manage your weight and health for the rest of your life? Not as much.