I've known people where it worked out great. However, one can always question of it was the hypnosis or the believing in it that caused it working. Kind of same thing though.
Whaddaya want? Magic? Hypnosis focuses the mind and makes all manner of things possible. If just "believing" that it works does the same, we have a result. :)
I haven't smoked in 40 years. However way back when I was trying to quit I did seek medical hypnotism. It did work for about 2 months. After 2 months I found myself in a very stressful situation I unconsciously picked up a lit cigarette (belonging to someone else) from an ashtray and took a puff. That was the end I was hooked again. Fortunately I went cold turkey a few years later and did finally quit.