The flat is managed by an agency but this does not cause any disadvantage. On the contrary they are very polite, helpful and responsive in communication and do everything to make your visit a positive experience.
The location is perfect you can reach every key tourist attractions on foot. The flat is in a charming old house in the historic old town, very atmospheric. There is no traffic around however there are restaurants and bars nearby even in front of the house on the nice little square so expect some noise in the evenings. However around 11pm everything goes quiet so you can sleep well.
Do not expect high-end interior and equipment but it perfectly serves your normal needs. If I were to visit Lisbon again I would stay here again.