I have - to give up smoking.
It's not so much about being in someone else's control as someone else helping you to re-prioritise conflicting thoughts (eg. the wish not to smoke vs wanting a fag).
Actually i don't think it would have worked had he not been a friend who was in training and using me as a test case, but that's not unusual - the prices are quite high and some hypnotists deliberately re-arrange appointments just to give you extra motivation.
It did alleviate the withdrawal symptoms though - it wasn't too bad at all. and though i smoke again now, i have permanently lost one of the things i hated about smoking and that was my total and utter reliance on it. I never used to be able to go on an plane without being a nervous wreck at the other end - i can now.