My wife and I had dinner at American Seasons on two occassions this past week. By far, the food is the best on the island. Try the foie gras trio, the warm octopus, the flounder, and the oysters; all terrific.
Everything on Nantucket is 20% more than Nantucket prices, though the service is never on par. While the food and setting ( romantic / candlelit ) is quite charming on site, the service is cold and uneven.
But while in nantucket, whats to worry about.