You cannot reserve Cabanas in advance online right now, although that feature is coming. Currently, they are $499, and you can't reserve until you are onboard. Suite guests get priority reservations.
I was just given a tour of the cabanas on the family beach last week...before I saw them, I didn't think I would be willing to spend the money. But a cabana would be great to have...you don't have to worry about guarding your lounger or someone taking it when you go eat, you don't have to deal with latecomers dragging loungers in front of yours. It comes with drinks, snacks, sand toys, sunscreen, and other little perks.
The main problem I see with them is that they only allow 6, and they are very strict about it. The downside is two couples each traveling with two kids can't get one together...I really think that Disney should consider making it at least two quad cabin occupancy. The whole thing is constantly changing since it is new, so the capacity could change.
I would avoid Cabanas 14-17 on the family beach...they are really far from the bathroom and also on a rocky part of the beach where no swimming is allowed.