tl;dr: A great choice. Good location, pleasant host, and good quality room (with a balcony!).
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Communication: Timely and responsive. Arthur speaks English and Portuguese while his mother speaks Portuguese. I only speak English, but we were able to communicate fine.
Arrival: Easy. Arthur was responsive over the app and provided instructions to reach to his apartment. He oriented me to the room.
Host: Friendly, accommodating, and helpful. Arthur oriented me to the room and answered my questions. We generally did our own things (works well for me), and interactions were pleasant.
Space: Sizable room containing a bed easily fitting 2, shelves by the bed, power strip (nice touch), wardrobe with hangers, nightstand with TV, and balcony (!). The bed is spacious and relatively comfortable though you can feel the springs. The balcony is easily my favorite part with a table and two chairs and a view into the city. So nice-- my friend and I spent our nights chatting while enjoying the outside. Bathroom is spacious though the tap was not fixed to the sink, which was quite annoying. Kitchen houses the washer and is well-appointed. There may be other guests (though I wasn't always sure who was family vs. other guests). It wasn't an issue. I did feel the room was sometimes uncomfortably cold, especially in the morning, though Arthur did provide an extra blanket, which was helpful. The apartment and room could have done with some vacuuming. Overall, I thought the apartment was adequate for our needs, and I was very happy to have the balcony.
Location: Excellent location. Restaurants and public transit nearby. Several sights within easy walking distance. Can also get to key areas (Alfama, Baixa, the waterfront) with 30-60 min walk (or faster by metro - station a few min away), which my friend and I preferred and did without issue.