I think we booked a few days in advance two years ago. A storm was approaching so they called it a day just as we were ready to go on. However, right there we were able to make reservations for the next day.
Because of the price I imagine that it's not such a brisk seller that you need to book it weeks ahead of time. However, if you want to go on a particular date and at a particular time (the boat goes out either every half hour or every hour -- and on our trip out took four different parasailers) I'd call in a few weeks ahead.
Was it worth it? It was an experience. It's not as thrilling as it appears and you don't go as high as you do with other parasailing stints out in the Caribbean. However, you do get a unique one-of-a-kind view of the Magic Kingdom and that was worth it (for me, at least).