We've rented one twice on the Family Beach and enjoyed them both. As you say, they are quite pricey, so the value for the money is really up to the individual. The only things we didn't like was that the swimming area is more rocky than the rest of the beach and that the tide was going out both times, so by early/mid afternoon the water in front of the cabanas was pretty shallow. I could go all the way out to the rope (probably 200 - 300 feet from shore) and the water was only up to my knees.
They were both well stocked with sodas, water, snacks and sunscreen. The wet towels in the refrigerator were nice because it was hot. It was nice to be able to go to Cookies Too and bring lunch back. We were in numbers 4 and 5, so it was only a short walk to get something to eat.
We're going on the Dream next week and have a cabana booked at Serenity Bay (now that DS is 18 and can be there with us).