I'd definitely stay here again if I were in Lisbon. Great location! So many nearby restaurants and things to do! The flat itself was also very spacious and looked like the photos. As mentioned, the flat does not get much natural lighting, but that was actually great for those of us that required absolute darkness for a good night's sleep. Mario was helpful and responsive, and everything was as described. He also gave us a grand tour and helpful tips and a map.
The only things I would note: Watch your step. These steps aren't marked so you are either stepping up or down from the bedroom/bathroom/kitchen. Also the wifi does not reach the furthest bedroom, but it wasn't a big deal since I was only in the bedroom to pass out after a long day. The bedroom closest to the street is a bit noisy, so if you are traveling with a group, I'd recommend the heavy sleeper stay in that room and the light sleeper sleep in the furthest room. Otherwise, I'd highly recommend Mario's flat.