My local hospital don't actually do any antenatal classes now which I think is a travesty!
I did the NCT classes with my first and also a refresher with my 2nd child. I haven't done any this time around and do feel less prepared.
The NCT one is great for meeting mums in the same boat as you. And that is invaluable. I don't think I could have coped without my circle or mums all going through the same things at the same time as I think it could be easy to feel isolated with a new baby. In fact one of the ladies I met is now one of my best friends and I still see at least half of the class weekly and have done for 5 years. I don't think you get that from the NHS ones. The NCT arrange a postnatal coordinator who hosts weekly coffee until all the mums have had their babies and then they leave you to it to organise it yourself.
I also didn't feel, unlike FatSeal, that they rammed natural childbirth down your neck. They also went through c-sections, epidurals, induction etc. They taught us how to change a nappy and breastfeed or bottle feed. They also took us on a tour of the local hospital.
What they did teach me was that labour isn't how you see it in films - it is very unlikely that you'll wake in the night as your waters go and turn to your H and say "honey - I think it's started!!".
I know that not all labour goes to plan and I had an open mind about what would happen and would go with the flow - and the classes allowed me to feel like this. But they also gave me the confidence to challenge the assumption of the midwife that I would be booked into my local hospital (I am terrified of big hospitals) and instead I booked into a birthing centre 40 minutes away. They made me realise that I could "do" childbirth and that I didn't have to accept intervention without question. If I had gone to my local hospital they would have automatically burst my waters on arrival at 5cm gone which would have ramped up my labour and caused more pain. Instead my first was born with them intact and with my second they went 2 pushes before she was born. Without the NCT classes I never would have made the decisions I did and been in control of my labour myself.
So I would suggest definitely going to the NCT ones and if you have NHS ones as well then do them as a secondary class.