I am going to NCT classes at the moment, though oddly enough not as a pregnant woman (though I am also pg :)) but because I'm training to become a breastfeeding counsellor with the NCT so need to observe an AN course as part of my training. I didn't go to NCT classes when I was pg with my DD as we moved a couple of hundred miles when I was 32 weeks pg and felt there was no point going where we lived before, as there was no point meeting people there, and they were full in the place we moved to.
As an observer rather than a participant, I think they're fab so far and am really enjoying them. I'm hoping to persuade DH to go to some later in this pregnancy, seeing as we didn't go to any last time, even though we've done it once.
The main reason people seem to do them is to meet people in the area which is definitely a bonus - particularly when your babies are tiny it does help to know people who are at exactly the same stage. My mum went to NCT classes with her second baby and is still in touch with most of them now (that baby is now 32 and a father of two :)).
The other thing that I hadn't really appreciated is that it also means that the DHs/DPs also have to go along and pay attention - I did buy DH a few books aimed at men and he obviously picked stuff up from me but I think it would have been really useful for him to go along and learn some stuff.
It probably does depend to some extent on the teacher - the one who does this is fab, she's a MW so knows her stuff and is really friendly and approachable. I have actually learned a few things
The group also seem really sparky and nice and I'm a bit sad I don't get to be friends with them all at the end 
Can't remember what else I was going to say, bloody baby brain 