In general, our stay was very pleasant and we totally recommend this place.
Good points: the apartment is at a superb and very lively location, ideal to know all the sightseeing points of the city by foot. The attention from the agency managing the apartment is fantastic. They responded all our inquiries within minutes and provided everything we asked them for. The amenities and household appliances are amazing and in a great condition (oven, fridge, microwave, dish washer...). Many great and affordable local places to eat a few steps away (especially Duque and Fernandinho) but it is a quiet street and not noisy at all. Decoration is good and almost everything seems new.
Not so good points: natural lighting and ventilation in the bedroom is inexistent, but if you just sleep there is fine. Stairs are very steep and narrow so be careful, especially for elder people. Just a floor anyway. The bathroom is small and you need to step in the shower to be able to go out from it.
To improve: the check-in at a different location is inconvenient, especially when you arrive with luggage to such a hilly city as Lisbon. There should be a lockbox with a code or something directly at the apartment to avoid the keys pick-up. Internet connection was worse than expected (speed less than 10 Mbps) and a bit unreliable with bad weather. The pillows are not good enough and the bed is a bit quirky and does not feel very stable. Sofa fabric was full of stains and we had to cover it with a blanket to feel better. A good cleaning may be recommended. Maybe a bit chilly during cold days but heaters are provided so it is fine.
Tips: contact the agency directly through their web to save some platform fees. They are just too much. Go to the apartment by Uber from the agency office. It is cheap and you will save the heavy stairs. Do not leave without eating lunch time menus at the restaurants mentioned before. Portuguese home-made cuisine at its best!