I went to Mindful Mamma hypnobirthing at about 28 weeks. I kept joking about how I was going to get brain washed, but truthfully I had been quite anxious about labour and pain, but wanted a non interventional labour and would gave thrown any kind of money towards anything that would help.
The full day class was incredibly helpful. We went through a bit about anatomy and hormones, and his important the balance of oxcytocin and adrenaline is during labour. Once I understood the physiology, I became really open to the breathing and visualisation techniques that were taught to help you relax and keep focused. The instructor also went into a lot of detail on how the partner can help in creating the right environment with you. We came away feeling much more positive, and I am really looking forward to the birth now and not worrying about lots of interventions rather than feeling scared.
It's a chunk of money, but the class is 1/3 the size of the NCT signature class I went to, and a warmer, more comfortable environment. The two classes have overlap but are quite complimentary.
We also received a book, links to mp3s that help me relax and fall asleep to, and a ton of links on positive birth experiences. The nice thing is that they don't promote one ideal way of giving birth, but teach you techniques that you can use in all circumstances in case your birth plan does not go the way you want.
So I'd say, if you know what you are doing and already feel positive about pregnancy and giving birth, then you may not need the class (though I'd still recommend it). But if you are approaching it with some trepidation, then it is absolutely superb. I'll also say that my partner was a bit cynical of hypnobirthing, but he cane away very impressed and encouraging me to keep up with the exercises.