I tried it and really do not advise any sort of "dieting" while breast feeding. Weight watchers (as most diets are) are based upon your own caloric needs and intake. However, when you BF, your not just using calories for yourself, but for you baby as well and tend to burn an extra 500-1000 a day just by breast feeding alone. So you will need to eat more than what weight watchers would allow in their points system.
Right after I had my first, I thought I would amp up the weight loss and do weight watchers (been doing it on and off for 10+ years and always use it to drop a few pounds now and then) and my milk supply was nearly crushed within the first week. I finally had to just accept that while breast feeding, the best thing to do is stick to a "clean" diet of healthy fats, lots of lean meats, fruits, veggies, and whole grains. I stayed away from refined sugars, white flour, and anything that basically came from a box and just ate when I was hungry...probably close to 2500 - 3000 cals a day and I was able to drop all my pregnancy weight + an extra 20 lbs (50 lbs total) by 8 months PP.
If your not significantly overweight, I would just suggest to eat clean and eat often. You may be surprised how fast the weight comes off just through BFing.