I will go out on a limb to say (opinion only here, no science to base it on) it's slightly more successful than just the placebo effect.
Simply because one can use hypnosis as a means of aiding conditioning. In cases of compulsion or addiction there's a form of a reward system going on. Now hypnosis will not break the physical addiction to a compulsion (if there is one involved) but it can be used as a aid to alter the reward system that's causing the complusion.
But then again, anyone can do that with enough desire, concentration and motivation. I believe hypnosis can be a tool in aiding someone to do that. Maybe if the person is suggestable enough, it would be enough to help.
Again, opinion and conjecture only.