I studied with Marie on her training course when I was pregnant (I was a therapist back then and we are the worst patients ever).
I'm quite practical (so knew all about what pain relief was available and wasn't going for a 'natural or failure' birth). It never says that it guarantees you any birth in particular - just that you get the best out of any situation. If you get the books and cds if so use the American recordings as the British accent just sounds weird. In hypnosis some wording sounds odd ( it there is a reason for it) and somehow an American actor accent gets away with it better!
The birth itself was very boring and I didn't feel the need for any pain relief - I was curious about the air and gas but couldn't be bothered to walk across the room to try it to see if it was like the dentists gas.
I ended up on a drip to 'get things moving' as lazy bones baby was taking his sweet time (he is part sloth) which I was warned would make the contractions more severe. The nurses didn't believe that I was in labour when I arrived, nor that the baby was crowning as I was 'too chilled'.
I ended up teaching it and it does set you in good stead if things don't go 'to plan'.
If you are calm and clear headed (and know what questions to ask) when things start going 'wrong' then you have (or feel that you have) some control over what is happening to you and your baby.
I had one mum who ended up having an emergency c-section pretty early due to health problems with the baby. Dad called me to say that they used the techniques on the course to keep calm and focused, discussed options with their doctors and the hospital and it stopped them going into a blind panic. Another was a friend abroad who did the course there and had a back to back birth using HB - she said it stopped her being scared and panicky. SiL had 2 very large babies without any pain relief and said that giving birth was 'fine'. The feedback I got from parents was that it gave them condidence that they had tools on the bag for whatever turn tool place on the labour ward and that the main thing was remaining calm and relaxed, and feeling in control.