Hiya,
I did hypnobirthing classes when I was pregnant with my son nearly 4 years ago now.
I think there are pros and cons to them and I think the cons probably very much depend on who you do the classes with, so my list of cons are mainly very specific to the lady who ran my classes.
Pros:
I went from being terrified of the birth to actually looking forward to it
I was very calm throughout labour, the midwives all commented on it
I felt I had a real understanding of how the body labours and the different stages (The Marie Mongon book is excellent)
Cons:
The lady who ran my classes had her head in the clouds a little ...
She advised NOT doing a hospital tour as she said that if we did that then we would be thinking about that and not focussing 100% on the home birth (a bit like cosmic ordering).
We did do a hospital tour and when my ds wouldn't come out after 2 hours of pushing at home and we got transferred I was very glad I had as I knew what to expect.
She assured me that if I did the hypnobirthing techniques then labour DOESN'T hurt (she had had 2 c sections so wasn't exactly speaking from experience) so I was quite shocked when labour started to hurt!
She was very dismissive of the NHS midwives but I found them to be excellent and interested in hypnobirthing and they used the terminology like 'oh fab you are already 4 cms!' rather than 'you are only 4 cm'
Oh she advised us not to do any other childbirth classes (in case they talked about 'pain') but I did both the NCT and NHS ones and made friends through the NCT ones and was reassured at how kind the NHS ones were.
My labour with my son was back to back and so I had to walk the whole time and couldn't lie down and do the visualisations and we hadn't covered what to do if you just had to walk up and down!
I was very angry with the lady after my birth and was too angry and upset to contact her after the birth. I felt that if I had followed her advice I would have been less prepared for what happened and she was overly negative about the NHS staff.
Sorry, this has turned into a bit of a rant! Just getting a few things off my chest 
But, when you look at my list of pros, it really was helpful in many respects. So my advice would be, do the classes but make your own decisions as well with regards to whether you do other classes/hospital tours etc.
I think I lost faith in the lady who did our classes when she showed us a spoon she had bent using the power of her mind ... says it all really!!