My girlfriend and I stayed in João and Vera's Green Windmill Apartment and it was an essential part of our enjoyment during our visit to Lisbon.
The location is quite central without being subject to the club or bar crowds, with most everything in the old quarter being at most a 20 minute walk away and there are busses and the metro really close by.
The apartment itself is clean and well appointed, with a good kitchen, comfortable bed, good water pressure and plentiful hot water.
We didn't get to meet João but Vera was very warm and welcoming, providing us with lots of detail about what to do and see in Lisbon and its surrounding area. She gave us several tips which were invaluable and which made our visits to some of the sites so much better.
So yes, we would heartily recommend Vera and João as hosts and their green windmill apartment!