re: What are the crowds/atmosphere like for the French Quarter Festival?

We usually park deep in the Marigny and walk in. Grab food by the mint. I also have never really had a problem getting food at the places I like in Jackson Square (Crawfish Crepes) and can always get a decent places to stand for the stages at the riverfront.

If you want to stake a spot, people do that as well.
Ten+ years ago was the time to go. It was not as popular , hell I think the Strawberry Fest was the bigger draw that week. (always same time every year both fests)
Still for great music and good food, it's a must do. I skip Jazz Fest, that's the one that got too big IMO.