The statistics I have heard is 60-70 percent do keep the weight off.
BUT if you dont have a plan and some changes you are willing to make to your eating and fitness habits, YOU WILL PUT THE WEIGHT BACK ON. I cant stess enough that if you dont change the habits that got you to the weight you are today, if you dont make some mental changes and have a plan on how you will take care of the new you, you will gain it back.
I have taken close to 60 pounds off. When I got off my last round I didnt stick to the changes I had committed to. After about a month of going back to my old habits the pounds started creeping on. As soon as I began watching my portion size and exercising the pounds came back off. Its as simple as that.
Hope my story helps you