Depends on where you live. Last time I found you had to book really early as everything got really booked up.
Like everyone else says, the NCT will tell you when the most appropriate classes for your due date are.
I'd also recommend your hospital classes if you are able to attend them.
Our NCT classes were very good for the 'holistic' approach, and very educational on natural birthing, breast feeding, and thinking how to involve your partner, how the arrival of a child will impact your life and relationship and such. They also covered interventions and pain relief, but not in that much detail really (I'm told some NCT teachers are so pro natural birthing, they don't cover these topics at all).
We found our hospital classes really informative on their own policies- ie at what stage you should call if you go into labour, when they'll let you use the birthing pool, when they are likely to recommend inductions if you're overdue, what happens if you need a c-section etc.
NCT was also great for meeting people of course! I made some great friends.