It is what it is..
Fat Boy is what it is, an authentic 50's style drive-in, not one built to look like one. The food my not be the best in town, but it's exactly what you would get if you went to a drive-in in the 50's, cheeseburgers, fries, onion rings, frappes. They also have some seafood that is really quite good. I enjoy the lobster rolls, crab meat rolls, fried clam and shrimp dinners.
This restaurant has been extremely popular with both locals and tourists alike for decades, so peak hours are very busy. It takes a while for a car-hop to get to you, and then a while for you to get your food. I avoid these peak hours. If you go a little before or after peak times though, the service is quite efficient.
If you're looking for the best food in town in an elegant environment, this is NOT for you. If you're looking for a really laid back experience, and classic drive-in food, it's a great choice. Certainly better than the fast food options in the area.
By the way, there is indoor seating for those who do not want "greasy fingers" inside their car.