Weight Watchers is made up of two programs, Flex and Core.
Flex is based on a point system, where all foods are given a points value. Based on your height and weight you will be given a daily points target and you will need to eat food each day totalling your daily allowence. You are also given 35 flex points that you can use any way you want during the week; so if you want to eat 5 points more a day you can or you can save it for something special within that week. You have to eat all of your daily points, but you can opt to only use some or all of your flex points. You also can earn activity points by doing exercise and again, those you can use all of some.
Core is based on a group of core foods, mostly lean protein, fish, whole grains, fruits and oils. You can eat what you want off of this list (which they provide you) and you eat until you are satisfied. You also have the 35 flex points for the week and acticity points as stated above.
I have done both plans and have seen success with them both, it is purely a personal choice based on your lifestyle. You can also incorporate both into your life once you get going.
I have successfully lost 65 pounds with WW, it took me 18 months to loose. But as you can see from my siggie, I gained 25 back, hated it and lost it again. I have been at my goal this time for 11 months and I am determined not to go back this time.
As for costs, I always like the prepaid plan because it was cheaper than the weekly rate. I haven't paid in a year so I am not sure of the current fees, once you make lifetime you are free as long as you stay at your goal. I want to say it was around $39 a month on the prepaid plan. Check with weightwatchers.com, they should have the current fees available.
I hope this helps and I hope someone else out there could add a little more.
WW is a great program and it does work, it is all about choices and lifestyle, but it is totally doable.