Hi! So I started this in the summer of '07 (got derailed by school and aquiring a boyfriend who eats too much fast food), and here's what I experienced in my 3 weeks of being good and on plan before I succumbed to Del Taco and being lazy.
First off, I lost 15 lbs and dropped one pant size (from a 16 to a 14) in the three weeks. The first week was tough, what with the lack of salt, eating so often (I'm used to eating 1-2 times a day, and never cooking), and drinking water. After I got used to that, and cooking a weeks worth of food at a time, it was pretty easy. Basically you just eat different proportions of protein/fruit/veggies/carbs per meal. I lost the 15 lbs without doing the 18 minute resistance routine, and by walking for an hour only like 2 times per week. What really ended up derailing me is the prep, but if you can work it out and devote 2 hours on a Sunday to cooking for the rest of the week, and can handle making more protein/carbs/whatever on a Wednesday if you need to, then if you're determined you should do fine. Another awesome thing is that they have a very cool forum where you can exchange recipes and whatnot, and the people in the commercials are still members posting on the site (I've talked to a few of them, even), and people there tend to be pretty cool.
I'm such a fan of the program that I'm starting it again on the 19th for what I hope is the last time.
As for it being a long term sustaining program, they have a subsection of it called "Living Lean" where you still eat frequently and everything, but you can add in salts and sugars and refined carbs in moderation. From what I've seen, it's very much a "what ever works for you" part, as some people can have a cupcake a day and maintain, and other can eat too big of a banana and gain. The kit also comes with a restaurant guide, so you can make more educated choices in terms of ordering out, and the forums have a feeling very much like getting back on the horse if you went crazy at a family barbeque.
Anywho. I'll be starting again soon, and good luck to those already on the plan!