We rented a cabana for 2 days on our trip in June, last year. Found it well worth the price. The pool gets very crowded and there are not a lot of umbrellas for shade. Having a place to be without having to fight for a chair and shade made it worth the money.
We spent 2 full days just hanging at the pool and the time flew by. Previous posters are correct. It is first come, first served for choice of cabana locations. The sooner you are on the list the higher your priority.
The first day we were booked we had 2nd choice and the second day we got 1st choice. I do believe that if you cancel the day of your reservation there is a charge but you can cancel the day before. They maintain a waitlist for the cabanas and just add someone the day before.
The cabanas have drop down sides and an interior fan, tv, refrigerator and safe. They are close together but you can lower the side curtins and not really be bothered with the people next to you. That being said, we preferred the quiet pool, as at the time there were only 2 cabanas there and it was outside of the pool traffic flow.
If you can afford it I highly recommend the cabanas.