I would say it will work for everyone, but some might have to work much harder than others. This has sometimes been the case with those who basically cannot say anything at all. It is all mind related and if you start working on it you will eventually reach goal. You might want to read some pages from John Harrisons Redefining stuttering too (free book). Read the articles about the Hexagon and the Hawthorne effect. That will give hopefully give you some information about things you can focus on improving and explain why it works and "might not work" for some.