I happened to see this question in a forum so thought of writing my experience.
When I wanted to attend antenatal class my hubby first question was "Did my mom or your mom attend such class?".
It is true that neither did my mom nor grandmom attended any class regarding child birth or child care. But in olden days, people lived in joint family or community, there were chance of seeing and knowing from other pregnant women or babies. In today's nuclear and secular family, exposer to people of different age group has become less unless its your sibling. Hence there is less or no knowledge on pregnancy and child birth.
I can read books or there are hundreds of articles on internet. Why classes? The Antenatal classes teach you pregnancy exercises, information on pregnancy and labor and child care. Most books tells you about just one topic. In the classes, you can meet other pregnant women. The discussions might lead you topics that might not have crossed your mind.
Finally, Is it useful?
Yes. You learn exercises in the correct way which is very important since your ligaments are loose and any wrong positioning can cause injuries. You know about labour process what they do and what to expect. Your diet is analyzed. Your doubts are cleared. You get good tips on child care. You get information from other pregnant women. So if you can join an antenatal class, do it without fail.
When to start antenatal class? Ideally 20 weeks or after anomaly scan is the recommended period. But don't delay after that since you won't be able to sit for a long time or make use of the exercises fully. Always consult your gynec before enrolling.
What to look for?
Look for classes which allows you and your husband. Take your hubby along. Look for classes that cover both child birth and child care. Look for reviews regarding the class. Talk to the instructor, is she soft spoken and answers your queries? Look for a clean place.
