Highly recommended. The flat is incredibly central, perfect to explore the heart of Lisbon; and has the advantage that you can walk to many places without having to climb a hill, which is rare in this city. Also, close enough to Cais do Sodre station from which you can easily take a train to the beach areas.
Moreover, the flat is a place that makes it nice to spend some time in. The common areas (bathroom, living room, dining area, kitchen) were very clean, mostly well decorated and with a lot of natural light thanks to many large windows. The bedrooms are decent. Beware though that one of them doesn't have a window and there is no A/C; however, a fan is provided.
Finally, Sofia is a great and dedicated host, who will offer her help wherever she can. We really enjoyed our stay here and would come back anytime.