I did the classes with my first. I was well read, knew most of the info they gave us etc but it was well worth going. It gave us a chance to ask questions about the hospital and what to expect from a midwife who worked there and could give us some insider info. It's one thing to read it in a book or hear other people's experiences and an entirely different thing to go through it yourself. Also, we did things like getting all the dads to write down their expectations or experiences and compare them to the mums - things like that can help to make sure you're on the same page.
I also met other mums and we still catch up every week for play group more than 3 years later.
Even if you just take one thing away from the classes, it's worth doing. If you do them and don't get anything out of it at least you know you haven't missed anything.