Though we often opt for taxis, we found the Alibus was fine to get to Naples airport (and vice versa) as long as you don't mind spending a little extra time. No need to pre-purchase a ticket- pay 5 euros each to the driver. At the airport we waited about 30 minutes to get on a bus back to the train station (having taken a side trip to catania). The people who elected to purchase alibus tix in the airport were far behind us.
Food bonus- if you have extra time at Centrale, the food court has a branch of Florence's well'-known All'antico Vinaio sandwich shop- second floor.