No, coming off Weight Watchers, I didn't find it hard to follow, but some people do have difficulty right at first. The diet makes you VERY aware of what you're eating and what you should (and shouldn't) be eating. You'll learn a LOT. I've always thought I was pretty well informed, but I've since been seriously humbled--especially on the issue of sodium in my diet. Yikes!
The program begins with a Conditioning phase, which builds your metabolism, and after that the restrictions tighten up gradually, so you have time to learn and find foods that "fit" the program. But no, there is no "prescriptive" diet. You work with the foods you like, the foods you normally eat. You'll probably have to modify some, toss out others, and add things you've never eaten before. (For me, one was avocados--yum!)
My thirteen-year-old son has just started and is muddling through, with a little supervision from me, so I'd say anyone who's determined can do it. Those who make it past the intitial learning phase seem to be, for the most part, hooked for life. It does take a committment, though, especially at first. I spent as much as an hour a day in the first week or two, learning. That was a struggle, as I'm a teacher, but you'll learn fast. Now it takes ten minutes a day, tops.
There's a short overview of the program (e-book) that will give you a lot of info and a terrific support group that will help with any questions you have. The guy who designed the program responds to most of the questions personally--or you can always e-mail me.
I correspond with several people about the program simply because I think it's worthwhile and an option I wasn't previously aware of. And once we get in touch, we all want to know how everyone else is doing.
A lot of people who do Body for Life and Body Rx find their way to physiquetransformation.com because it is more precise. A lot of former WWs end up there because they plateau, as I did (usually because of too much exercise, not enough fuel). People who plateau with the "Body" programs or who start to exaggerate those fist-sized portions (as I KNOW I would), eventually wander over there. It just depends on your personality, I think. Personally, I like the security of tracking my diet "exactly." I know that I'm getting all the nutrients I need and that what I'm eating is not only helping me lose weight, it is also helping me to get healthy. Other people don't like the program for the very same reason I do.
As for gaining weight, it's possible. But you WILL lose it! Personally, I didn't gain, although I was absolutely terrified I would. In fact, if I had seen any other possible option, I would never have tried the program at all. I have a thyroid condition that makes weight control much more difficult. I was so afraid I'd gain 5 or 6 pounds then never be able to get it off. I saw myself buying new "fat clothes," losing self-confidence, etc. But that just doesn't happen. I've never talked to anyone who gained during Conditioning who didn't eventually lose it all plus during Fatburning. As long as you don't give up, you'll succeed. It's more psychological than anything--a leap of faith. At least that's the way it was for me. A lot of people panic, then come back later with a great big "Phew!" And there's a lot of support for those of us who do. The program is great for people who want to lose that stubborn "last ten pounds" and for those of us who exercise regularly because so many of us are undereating.
Well, I hope this answers your questions. And good luck if you decide to give it a shot.
Shari