I read the Mongan book a couple of times when I was expecting DD and listened to the Howell natal hypnotherapy CDs a lot once I was on mat leave - guaranteed afternoon nap every time!
I would say they enabled me to be pretty chilled and unfearful of birth during the end of the pregnancy and helped me relax and go to sleep etc so I would do it again for that alone.
Once in labour I remember lolling around in the birth pool drugged up on gas and air and semi comatose as a result, and once or twice in moments of lucidity snarling at DH "I'm glad we didn't fork out on an actual hypnobirthing course, the lying bastards, it bloody hurts!" The MW gently intersposed, well you practically are hypnobirthing... I wasn't tbh, I was frightened and drugged and exhausted. However it's probably worth adding that I had a v long labour so by that point hadn't slept at all for 3 days and 2 nights, and was stuck in the 8-10cm zone for 4 hours which was when I lost the plot somewhat. It was all very calm and manageable up until that point and yes the breathing and calm thing stayed with me up until about 7cm.
I didn't do the whole "breathe the baby out" thing - even giving it fully welly massive pushing efforts it took 2 hours to push the wretched child out so I dread to think how long calm breathing might have taken. That said, when they examined me and said I was 10cm, they said to push when I had the urge. I didn't feel the urge but thought ha, they won't know that and I am fed up so I just started serious pushing anyway so it's entirely possible it would have been quicker/easier if I'd waited for the feeling (assuming it came that is!)
So, a mixed bag, but I will read/listen again later on this pregnancy and hope for the best.