Well, I started out on Flex and always had great success with it. When I got fed up with counting points, I went to Core and had success there too as long as I followed it. Flex was always great for its flexibility, and Core was always great for its emphasis on eating healthy foods and minimizing simple carbs.
Now I'm moving forward with WW's changes and following the Momentum plan, which combines the best of both plans - portion control and an emphasis on eating healthy whole grains, vegetables, fruits, etc. I really think they made a great improvement with the new plan. I mean, it's not wholly different from what we're used to with Flex/Core, but I was searching for a way to get the best of both the plans, and now they've given it to us!