If you live in an isolated place, you might benefit from knowing other mums due at the same time. Also the ones in my class were woman who I don't think I'd have met otherwise, 2 lived in villages near to me but we wouldn't have gone to the same toddler groups etc - nearly 5 years on, 4 of the orginal group meet up every couple of weeks, it's going to be harder after September when they are all at school, but we might just do meet ups with the younger ones as we've all had a DC2 now.
As for the actual information in the class, I don't think I needed the antenatal classes for me as I did just read up before hand, I think DH got far more out of it than I did. I'm a 'planner' and like to have information, so had been reading everything about pregnancy, birth, breast feeding, early days with a newborn etc since getting the 2 lines, so by the time I got to the classes, I sort of knew it all already, but DH hadn't and he really benefited.
NHS ones might be just as good for you,although they tend to be at silly middle of the day times, and yes, you can have time off work to go to them, if you live rurally, it could end up being a whole day out and just not practical. It might be worth seeing if there are any independent run classes near you or if the NCT do intensive weekend courses (which are cheaper and a lot less time commitment needed)