The apartment location is definitely the prime factor for choosing this stay. It is located in a lively neighborhood; a stone's throw from (and literally directly above) many things you might want to experience in Lisbon. Fado music, bars, restaurants, corner stores, monuments, markets, etc. And the train station is literally 5 minutes walk down the road. This is great if you arriving by train, planning a day trip out of Lisbon, or continuing your travels following your visit.
That said, the apartment could use some work.
Our first experience upon arrival was a strong sewer smell emanating from the shower drain in the master bathroom. We had to place a towel over the drain to staunch the stench so we could get some sleep and prevent all of our belongings from becoming saturated with the putrid smell. Big thanks to Gustavo for being on call to attempt to resolve this the very next day.
We visited during a summer heat wave and the only A/C unit was in the upstairs bedroom, which made this the only room that could be comfortably enjoyed. We admittedly spent most of our time outside the apartment. But if you intend to adopt a more "European" summer schedule (spending the hotter daylight hours inside, and going out in the cooler evenings) this might be something to consider.
You'll find cracks and paint problems throughout. The door knob to the bedroom stairs comically fell off in my hand a dozen times. The clothesline has been located under the dripping exhaust of the air conditioning unit. The bedroom blinds had to be rigged to stay closed, which was essential to keeping the bedroom cool.
Now, one might say "you're staying in an old part of Lisbon in an old building". I understand that sentiment, but also would point out that you probably would not attempt to rent or sell home in this condition without attempting make some cheap and easy repairs. The apartment is decidedly being treated as a rental and not with the respect of a home away from home. Also for perspective, our AirBNB stay previous to Lisbon was a beautifully maintained and renovated 400 year old cottage with modern amenities. While very small and simple, we had zero complaints.
In short, this was not our best stay, and not our worst. It is centrally located in a great neighborhood and provides the basics at a decent price. With a little care and attention it could be above average.