My wife and I stayed at Quinta Das Merces for 3 days in April 2011 while on our honeymoon. Especially in comparison to the other place we stayed, a larger chain hotel on Sao Miguel, this place has so much character and charm. The 17th century manor house has been turned into a comfortable B&B, plus there's lots to explore around the grounds. Alcino, the proprietor, is very friendly and kind. He helped us plan our touring around Terceira, recommended places to eat, and made our stay incredibly enjoyable. There's tons to see around the island, but after a day of exploring its great to come back to the friendly atmosphere and idyllic scenery at Quinta Das Merces.
The location is just off the main coastal road in between Angra do Heroismo and Sao Mateus. Its a significant walk into either of these towns, although the bus runs regularly past the hotel entrance and with our rental car we had no difficulty getting around. If you're driving, be forewarned that the entrance comes up FAST around a corner, but the sign is large and obvious so you'll know when you're there.
Our only complaint about our room was that it was consistently a little bit damp. Obviously this is just part of the deal considering how old the building is, and leaving the windows open solves the problem.
Overall a fantastic experience, and I highly recommend Quinta das Merces to anyone!