I had a wonderful 1 week+ stay in David's building in Barrio Alto. The apartment was clean and tidy, modern, and had Air Conditioning (!). David also operates the cafe downstairs (opens @ 6pm) -- my friends and I found it to be a wonderful place to start a night out in Barrio Alto. His staff were EXTREMELY friendly and I felt welcome eating there my first night in Lisbon (as well as the several times we stopped in to say hi to Juliana!).
There's multiple units in this building, so if you have the ability to book for you and your friends, I would do it. Our group enjoyed being in close proximity with one-another.
The only caveat that I feel I should mention is that the apartment is in Barrio Alto, with a thriving and active European bar scene ~5 days a week. I personally found this to be a major plus, as I was constantly meeting friends in the neighborhood and highly valued having a home base nearby. That said, if you are looking for a quiet place to recharge, some ear plugs would likely serve you well as the neighborhood can be boisterous into the early morning.
David is attentive, and highly responsive through Airbnb.com -- he will be happy to answer any questions you have, even prior to your arrival. I would absolutely stay here again, and I DEFINITELY will be returning to Lisbon in the not-so-distant future!