I have used a rental once, used a car service once, and the Mears shuttle three times. Yes, you do ha e to book it in advance. You can do it before you leave for vacation, or you can call them the night before, from BW. Either use a cc to pay when you make the reservation (and keep any confirmation numbers with you), or have exact cash at the bus.
I have been able to book a p/u time of 7:30.....allowed us to get to US/IoA, checked into RPR, and to the park gates by 8:30. Other times, we've been picked up around 8, and been there in plenty of time for opening.
When it was time to leave? We could have pre booked our return ride when we initially booked. But we chose to give them a call a few hours before we were ready to leave. We were then given a p/u time....we were usually able to choose between several times..
I have found this to be a great way to get there and back. It is about $20 pp, r/t, plus tip. A car service will run about $80 r/t, plus tip. Same with a cab. You could rent a car, but that's kind of a pain. You have to go get it, drive yourself to US/IoA, pay to park, trudge to the park entrance, then drive back, gas up, and return the car..
I would go the Mears route with just two people.