It was worth it for me. Good information, which would have been even more crucial if we'd had a C-section, as it really prepared us for what might happen. I made good friends in my group and all 9 of us are in frequent contact 2.5 years on. Some of us see one another 2 or 3 times a week, depending on work, etc. We also really enjoyed the NCT bumps and babies groups for the first few months. You don't have to be an NCT member to attend these though, and they're free. I think there are usually more non-NCT members present, anyway. Of course, you will also bond with parents you meet at the NHS classes, though (depending on the class) they tend to be over with pretty fast, so there often isn't the time to gel and make friends. NHS classes often have the additional benefit of including a hospital tour, though a lot of those are being discontinued.
I'm always shocked at the variation of cost for classes though. I suppose it has to reflect the booking costs of premises in various parts of the country. Have never come across a 'banding system' dependent on postcode before though. Just a subsidised rate for people on lower incomes. You don't need to join the NCT to do the classes, but I think you usually get a discount on the classes if you do.
Membership gets you into the famous, and useful, Nearly New Sales early (which can be crucial if you're after something specific) but as an NCT volunteer myself, I'm bound to say that you get even earlier access to the goodies if you volunteer to help out for the entire sale! 
Most NCT classes are well-taught and very balanced - there might be a few teachers out there who are still living up to the stereotype of anti-hospital, anti-drugs etc, but they are in the minority. The main ethos of the NCT is to offer parents information, not to tell them what to do - they assume you're intelligent and will make up your own mind. They are also very committed to putting forward evidence-based advice, but again, the onus is on you to decide what to do. If you felt that your classes were skewed towards anti-hospital, etc, you would be well within your rights to complain, and the teacher would be spoken to, maybe retrained.