800 calories, working full time and working out 5 times a week. If you can do it and feel great, yeah, it may have long-term damaging effects to your dieting efforts, yet you achieved your goal!
To some people, achieving the goal is enough. Going on a cabbage soup diet will work too. Eating nothing but hard boiled eggs and toast for a month, three times a day, will work, too. But here's the thing - doing something once doesn't make it your lifestyle and doesn't mean you can do it again and feel the same ease while you are doing it.
If you join WW, you will lose a ton of weight. Or not. But the goals are reasonable, the method (watching what you put in your mouth) is reasonable, and you can live with whatever works for you as you build your menus in WW.
More importantly, when you get to your goal, you can become a life-time member because you EARNED it (it's free, a kind of reward and incentive to keep at your weight) and you can use WW again to stay within a few pounds of your goal.
WW does not take the weight off of me. I do that with my eating and exercise habits. And I'm certainly not a salesman for WW (and I don't play one on tv, either!), just someone who thinks it's very rational and an antidote for people who tend to judge themselves by their success at efforts with extreme dieting.