Location!
We had a great time in Mafalda's apartment. It's comfortable and everything looks pretty new and it's a perfect place for a couple. But the biggest advantage is the location. Even being a couple of steps away from where the "action" is, it is still very quiet since it is on a side street and facing the back of the building. The apartment is in the middle of everything: cafes, transport (the touristic tram that takes you to Castelo de São Jorge/Alfama stops almost in front of the building), restaurants and etc.
I'd recommend Mafalda's apartment to any couple wanting to have a real "Lisboa experience" while being close to everything that matters.