Marius' place was a fantastic base for our stay in Lisbon! We ultimately picked this apartment because of the view, and let me tell you, it's even better in person! The location is perfect too, tucked into the old part of Lisbon and just a short walk from several major attractions: Castelo Sao Jorge, Alfama, the "downtown" area, the coast, several metro stops (including the Tram 28 initial stop), and a medium distance walk to the TimeOut market. It's also right next to a local grocery market where we grabbed some supplies and an amazing, highly-recommended seafood restaurant called Cervejaria Ramiro.
Note that as others have mentioned, you'll need to climb 4 flights of stairs to get up to the apartment. For us this was not a problem at all, and well worth it for the view.
The apartment itself was very clean and had everything we needed. Check-in and check-out were a breeze and Marius was very responsive before and during our stay. My one and only complaint was that I wish the apartment had air conditioning as it was quite hot in Lisbon during our stay (end of September) -- but the huge windows at least allowed for plenty of airflow and there was a fan in the apartment to help too, so it wasn't that bad.