Been there, done that with both them. And yeah, they BOTH worked.
Any good diet will work if you follow it. That's the big "if."
At the time I did Nutrisystems, my dh and I both did it and we both lost about 35 pounds pretty quickly. We both kinda "hit a wall" at that point. We just couldn't bring ourselves to consistently eat their food anymore. Of course, again, this was many years ago and the program has changed a lot. At that time, it was my understanding that you were allowed to eat the equivalent of 700 calories a day!
That is VERY low to do consistently. Now current wisdom is that following that low a diet for any length of time may contribute to gallstones etc. So it's also my understand that the Nutrisystem diet is no longer that rigid calorie-wise.
I know people who have also done well with Jenny Craig. It just depends on you and your needs. If having some more "pre-done" meals would help you stay the course, then Nutrisystem or Jenny might be just the ticket.
All that said, however, I prefer Weight Watchers. It's flexible in the diet plans, (essentially no food is forbidden but you are counting calories, which they translate into "points.") You also have the weekly accountability of weighing in if you attend their meetings. And lastly, I think it trains you better for "eating in real life" after you get off the initial "diet."
As to the earlier poster's question about what happens to your body when you start eating food "in the real world" again~~nothing. I honestly don't think it's a problem.
The deal is,
if you are prone to put on too much weight for whatever reasons, you are probably going to have to discipline your eating from here on out in whatever way you are able to. Otherwise, no matter what diet plan you use, you'll be doing it or something else again. Shepsmom mentioned this as well above and it's true. However, I do believe a "program" can help you lose weight. It gives you accountability (and that's BIG!) and helps keep you focused, etc.