It's working great for me! My weigh-in today was 3 pounds down. So far I haven't found it too tricky. I drink low fat soy milk instead of the regular soy milk which brings the points down a little. I also have a few favorite Amy's frozen dinners that I found to be pretty good on points. You have to be more creative but you can make it work. I don't know how the core plan would work with it because I haven't tried core with ibs yet, but the flex plan has been working great for me. You could even ask your weight watchers leader for some tips. If you talk to them about your special diet, they may be able to help you work it in with the weight watchers program.
You will notice that some will say that it won't work because they make you eat alot of insoluble fiber. But you don't have to follow EXACTLY what wieght watchers says. They are just guidlines. As long as you eat ibs friendly foods and try to keep them within your points range (if you are on flex plan) then you should be fine.
I've been on weight watchers for just over a year now and am 30 pounds down in total. I was diagnosed with IBS only a short time ago (a month or two ago I think).