Never had a birthing class, never will. I've read the Bradley books and the Birthing From Within book. I preferred Birthing From Within. This birth of twins is going to be my fifth birth (and sixth? Or is that 5th labor, 5th and 6th birth?) and I feel like if I don't know what I'm doing by now, no one can tell me.
To be quite frank, I feel like birth classes mainly exist for propaganda purposes, and I mean that in the best way. They are there to try to shape your worldview so that you know within yourself that you are strong enough to get through birth without pain medication, or to orient yourself to the hospital environment and interventions so that you can encounter them with confidence or at least familiarity. Propaganda or marketing isn't always about marketing a bad thing. But if you already know within yourself that you are going to be taking contractions one at a time and working to give birth to your baby, you don't exactly need a class in gaining that acceptance either, if you know what I mean? This is all sounding more harsh than I want it to, but I do worry that sometimes a class can be as limiting as is it helpful, like "birth: YOU'RE DOING IT WRONG! IF YOU PAID MORE ATTENTION IN CLASS YOU WOULD BE ABLE TO COPE BETTER WITH THIS PAIN!!!" I'm very wary of the One True Way of Birth.
Knowing that transition brings out self doubt in almost everyone is a really good thing to know, but I feel like being able to cope in labor is really more a question of mindset than it is about knowing this or that technique for working with the pain. A good doula or midwife will know the mechanics, and a lot of women will instinctually know what position will feel better with baby positioned X because they'll move around and see what hurts less, if they aren't tied to machines.
I can see how they could be really helpful for some people, but I wouldn't say that they are necessary by any stretch. It's a case of the mom needing to know what she needs in order to have the birth that is best for her and her child. If that means take a class, good for her for taking the initiative to have her needs met in this way. If that means a lot of praying or meditating to get her soul ready, great. The individual mother and child working with a good care provider will know what is best for her.