I joined WeightWatchers.com 3 months ago. I was ready to start losing a little weight and this is one of the 3 programs you hear about most often. Unfortunately it didn't work as well for me as I would have liked but that is mostly my fault rather than the program (darn it!).
I joined under the POINTS program. Everything you eat you track and roughly every 100 calories is about a point however 140 calories would be 2pts. The POINTS are based off the fat, calories and fiber. You can get meal lists for the week, look up restaurants, mark off how many glasses of water you've drank, how many fruits and veggies you've eaten and more in order to keep track. You can also weigh yourself one day a week and they monitor that on a graph for you and give you a start at each point of weightloss along with the percentage of weight you lost. It really is inspiring.
The only negative thing I would say about Weight Watchers is that even those who choose to do it online would like a little "real life" support without having to fork out the extra money. Even one meeting a month would help in the success of weightloss for those who need it.