Can I add to my previous post, I did find it a good way to meet people in a new area, so it seems everyone's experience is different!
Although I knew the mechanics of giving birth, and how to look after a newborn, from reading up on the subject, I found the classes did make me think about things I hadn't entertained a thought of - for instance CS. I never thought I would have one, when it was discussed in class I had a panic attack, which was a heads up for me to go and get some help (I have a hospital phobia) before the birth. In the end I had an EMCS, and the class definitely helped me prepare for this. Yes, women have been giving birth forever, but the question is whether you would benefit from the confidence that extra knowledge could give to you or your DP, and whether your experience could be enhanced by going to a class. Only you can decide that, and if you think there is no benefit then that is absolutely fair enough - having antenatal classes isn't the law!
Our local NCT branch is very small and everyone knows everyone else, which makes it easy to find out what's going on and who is good / not so good! One of our antenatal teachers ran the local NCT Bumps and Babies group (where I also made a lot of lasting friends) and she did the NHS funded antenatal sessions too, so there was lots of opportunity to find out if I was comfortable with the approach of the branch. As others have suggested, you could see if there was a similar local group and gather opinion on your local classes - they probably differ depending on the teacher.
You could also phone your local branch and ask to meet the teacher before you decide to enrol (maybe she could meet you before / after one of the current classes, for example?) so you can check that you feel comfortable with her and ask any questions about the course. It is fair enough if you are going to pay a lot for something!
I would also say that if you join a group and it isn't addressing the things you wanted it to, then tell the teacher ASAP and ask her for input on the areas you wanted covering. She should have the flexibility to do this.