I've never tried them, but I do have an opinion. I think they work as long as you are on them. Once you start eating as you normally would the weight comes back. To me it makes more sense to change and tailor your diet to healthier choices on your own than it is to pay big money for a company like Nutrisystem to do it for you.
I am about 25 lbs. heavier than I'd like to be. A combination of medicines I take for my medical conditions make me blow up like a balloon. I fought the weight gain by trying several different diets, but the weight never stayed off. I even discontinued some of the meds. on my own, against my dr's advice, because I hated the weight gain. Big mistake. When my DH found out what I had done he told me that he'd rather me be a little overweight than to be in pain 24/7. So now I don't mess with my meds. but I am making smarter choices about what I do eat and the weight is coming off.