I've taken Supershuttle several times and it has always gone smoothly. You can make reservations, but usually don't need to going from the airport to the resort unless you are going at a busy time. It shouldn't hurt to make reservations, because you aren't obligated even with a reservation. You usually aren't charged until you are picked up. You will need a reservation returning from the resort to the airport.
If I recall correctly, the cost of the shuttle is $11.50 each way per person (including taxes, excluding tip). I recommend against paying or committing ahead, because you may find that you want to switch to another company due to timing. There are several reputable companies picking up in the same area, and the one you selected may have just left) Similarly, there is no need to pay ahead for the return trip. Our last vacation, we decided that we wanted to return to the airport earlier than our scheduled reservation. (Heightened security alert) Another couple was in the same predicament. Supershuttle would not take us early, so we decided to split a cab instead.