We had a good, brief late-season stay in Edgartown this September. The launch service was responsive, and we got a nice welcome email from the Harbormaster. Weather prevented us from getting there for the first night of our reservation, and they were willing to process a 50% refund through Dockwa even though the policy specifies 10 days' notice. Only reason not to give 5 stars is the dinghy dock -- not a big enough facility, even late in the season.
Mystic Marine Fuels has the best gasoline prices we've found in New England. They're open long hours in the summer, and they give a $.10/gallon discount to Sea Tow members. Great fuel stop!
Late in the season, Nantucket Moorings isn't staffed on weekends, but the nightly rate is still high. The dinghy tie-up at the Harbormaster's town dock is still very crowded, and the restrooms are often locked. We had to call the town police to get the Harbormaster to come unlock the (old and poorly maintained) restrooms, or take the hike to the (new and well-maintained) public restrooms at Nantucket Boat Basin.