I really hate to be mr negative. But I have taken a few taxis, even though i always rent a car when there. My opinion is that many of these taxis are a death trap at the very least. They look as if they are about to fall apart . I see many that obviously need shocks or springs, the doors dont close all the way, they need mufflers ect ect. I took a few around Santo Domingo, because i didnt want to get lost driving anymore. But a trip from STI to Samana in the next taxi that shows up at the airport???????????? man that sounds like a crap shoot to me!. Again I know it was Santo Domingo, but i asked at the front desk of the nice hotel I was staying to call me a taxi, and make sure its a decent running car. A newer car.."do you understand me?".. he said OK no problem. Well a real junker shows up. the car is leaning to one side, and the floor boards are showing through. and the mufflers has a hole in it. Maybe I am gonna be carbon monoxided to death by the time I get where I am going.
so I would advise that you take up the persons advice that you call this guy JJ who has what most think is a decent car and let him drive you. He already has a good reputation. Or take the trip into Santiago , get some sleep and a good breakfast in the morning. Drive through santiago for an hour see the city some, and then take a leisurely ride to samana during the day so you can see what you are passing! The road is poor( especially up to San Fransisco De Macoris) and it could take 3 hours as it is. If you could do it twice you would appreciate the day trip and the sights more than the extra time at your final destination.
You could even choose the big bus as an option the next day from Santiago to Samana....big comfortable ride...........
best of luck...bob