I lost 45 lbs. on Nutrisystem about 15 years ago and gained it all back fairly quickly. It took 7 months to lose the weight and about three or four to gain it back because I returned to my old eating habits
In those days, Nutrisystem was formulated differently and I became very ill with gallbladder disease (like many others, apparently) and had my gallbladder removed. I think they now have a healthier balance in their foods.
Nutrisystem, or any other type of measured food system, helps you to recognize appropriate serving sizes. However, you can do the same thing by healthy food choices and an investment in an accurate food scale. You can also use 1 cup and 1/2 cup measures, etc. This would be a lot less expensive, plus you learn about portion control for the future.
Personally, I would rather do what I'm doing now - keep an online food diary (Fitday or Nutridiary are free) and measure EVERYTHING I eat. Sure, it's time consuming, but it's working and I'm finally learning how much I'm able to eat and not gain weight. I'd rather save all that money (for Nutrisystem) and spend it on a nice new wardrobe when I've lost the weight!