This is place is amazing! Claudia and Mário are great hosts, they were always quick to answer and giving us tips where to eat, what to see, even that we really don't like very touristy places.
The appt is very close to everything, walks at the Sé, sunsets at the ponte Luiz I is a must. Sao Bento station is 3 min walk, so if you are up to daytrips to Braga, Aveiro etc. the appt is perfectly situated.
The appt is really well equipped, you can find everything for cooking they even thought of little plastic boxes which were very handy for our picnics.
They even have a heater, which is very helpful on colder days.
The bed is very comfy, slept like babies.
The house itself is quite old with all of its advantages and disadvantages, so the walls are not to thick, so if you stay in the room a lot you can hear neighbours. But if you ever had one it shouldn't be a problem:D
I read in a comment that the flat is very noisy and it worried me a little. As living in a center of a capital I can say he noise coming from the streets are normal, like busy city normal, really can't do anything about is. Maybe Fridays and Saturdays are noisier because the whole city has a lot more people to accommodate and a lot of them coming to party.
All together this is a great place to stay, highly recommend it!