We have and loved it (tl & bb). Our location was at the end of a row of cabanas right next to a major walkway, so I would request to be somewhere away from a walkway next time, but even so it was wonderful. We had an attendantt that ordered and delivered our food, unlimited towels & bottled water (again by attendant), self-service refillable mugs, ice filled cooler, a locker, & most importantly the most wonderful shade and comfortable chairs.
As to when to book, probably depends on the time of year. I booked one of our reservations the day before, the other was a couple months out. With free dining requiring ph or wpm, I expect the water parks will be busier than usual in the fall. Your card won't be charged until you arrive and they have (iirc) a 24 cancellation policy. I would say that if you don't plan to be there all day, it's probably not worth it. But having a spot to completely refresh made the days so much more enjoyable!