I have around 20 years of experience with different kinds of speech/stutter therapy. It never helped me get more control over my speech longterm. I would estimate, based on experiences in last group therapies, that around 60% of the people who stutter improve (significantly) by following speech therapy. This is the group with a stutter that is more psychological based. Take the speech anxiety away and there they become more fluent. The group that has more severe neurological stutter is much harder to treat.