We did NCT. I wouldn't do it again.
On the positive side we didn't know that many people in the area and it meant that my wife had a support group of other mums on the run-up and following the birth of our son.
The dad's weren't all that sociable though, and the group as a whole was (predictably for where we lived at the time) a bit monocultural. Lots of bankers and the like with big 4x4s... we got some funny looks turning up in our VW van.
There was some useful stuff in the classes themselves but as others have pointed out, it's all a bit earth motherish. Militantly pro breastfeeding and natural childbirth.... that's fair enough, and certainly something which we were hoping for but at one one point our instructor told us in all seriousness that 90% of emergency caesareans weren't needed, which I actually thought was a pretty dangerous thing to say.
The result of this was that my wife felt under unreasonable pressure to have both a natural birth and to breast feed. As it turned out she had a really difficult labour, our son having moved into a breech position towards the end. She required an epidural and eventually an emergency caesarean which was just terrifying - it all came good in the end, but required a long time for both here and the baby to recover. We had some problems with breastfeeding initially, but stuck with it - although my wife (largely as a result of the NCT advice) felt really terrible guilt at the fact that we had to use formula top-ups. This, and the whole "competitive parenting" thing which developed in the group amongst some of the mums (some of the babies apparently slept through the night at 8 weeks!) probably contributed to a serious bout of postnatal depression.
If you do decide to go to NCT, my advice would be to speak to people who've been to the same group and see what their opinion of the instructor and the group was, and to take any advice with a liberal sprinkling of salt. It's your child and you who need to make the decisions about what is best for them and you.