We really enjoyed our stay at Mariana & Rita's apartment, and found it the perfect home to explore all of Lisbon. The location worked out fantastically for us - the quiet neighborhood allowed us to enjoy peaceful nights' sleep while still being close to all the great restaurants in Bairro Alto. When we wanted to head to Sintra or Belem, it was an easy walk down the hill to the Cais do Sodre train station and through the Time Out Market.
The apartment is tiny, but the design is so well configured as to allow plenty of space for lounging around on slow days with cute amenities and a great hot shower. As others noted the stairs can be a bit steep and short, but this shouldn't present a problem unless you have trouble climbing stairs already. We highly recommend the apartment as a great home base for explorations around beautiful Lisbon.