We're planning on renting one. We have 2 young children and won't be doing any other shore excursions. We know it's a lot of money (to us anyway) but it would make our day there much better.
When thinking of Castaway Cay (before kids) we thought of serenity, calmness, relaxation, paradise......
After kids I think of all the sand that's going to end up in the diapers, mouth, eyes, sun burns, etc.
Hopefully this will help me relax.....bring the kids in out of the sun, shower to wash off sand, cold drinks, place to change and put all of our stuff.
Plus we'll be celebrating our 11th anniversary (actually date the day before we cruise) so this will be our gift to each other.