From BOS there is a similar early morning flight to SJU. Have been pondering of taking that flight for my vacation to Puerto Rico. Problem is that in order for me to arrive at the airport at least one hour before departure I almost certainly have to take an expensive cab ride ($40) because the T does not run that early on weekends. That negates the "savings" of arriving early afternoon in SJU.
Remember, not everyone lives walking distance from the airport!