My wife used Nutrisystem back in the 90's to lose 20 pounds. It took her 6 weeks and worked very well. She actually lost about 25 pounds. She has maintained her weight ever since with proper diet and some exercise. We have a small in home gym with a universal gym and some weights. She speed walks and I still jog. I never had a weight issue, which used to really bug her. I can eat almost anything and never gain an ounce. I avoid sugars, red meat and alcoholic beverages.
In addition to eating just their (NS) food, you may also eat salads (with no dressing), some sugar free drinks, (if necessary), a 4-inch banana (too much sugar), and rice cakes to help with hunger pangs. The weight came off very quickly. She actually lost about 30 pounds, but we figured about 5 pounds were just water weight. Coffee with caffeine is also recommended as caffeine will increase your metabolism a lot. You should also do some exercise, like walking.
Bottom line is, yes, it works. But, if you cheat and think that you can get away by eating a pork roast or some meat loaf "just this one time", you are only kidding yourself. Once the weight is off and you learn how to maintain, then you may return to a normal diet with proper care.