We seriously enjoyed the being able to book fast passes as soon as we were in the park, even when it was for rope drop and the park hadn't even opened yet. We got way more done this trip than others and honestly I can see us going during a busier time now with MP and being able to ride as much as we really want with this feature.
Also, if you have 4 people who head to the parks in the morning and say after the afternoon break only 2 want to return, which for two days of our trip was my DD and I, while my DH and my DS stayed behind at the hotel or did something else, then all the MPs that you have booked you can use twice. We headed over and rode all our fastpasses twice! It was great.
Also, you can get a multiple attraction experience pass if the ride is down during your fast pass return window. One evening that my DD and I returned we had fast passes for GOTG which went down for not long it seemed but our fast pass turned into a multiple attraction experience so we rode RSR and then went and rode GOTG because there is no time limit on that type of experience.
I know it's $10 per person and I know it's likely to go up in price but it makes the entire experience so smooth and comfortable we will never visit without buying it unless they discontinue it.