Had a good week-long stay in the apartment, leaving early mornings and returning around midnights. That's the best mode of staying: the room is small, but has got everything you might need. Except maybe a kettle — when in Lisbon, you drink coffee. Perfect for one, may be acceptable for two.
One great advantage is that most of the city is in walking distance. There's a metro station, but you can get to the shore or to most parks in 15 minutes. A tram stop is like 50 meters from the apartment. And what a tram that is! The entire area feels very authentic, very portugese.
Stairs are indeed surprisingly steep, so you may have trouble lifting big luggage bags (or when being very drunk). The area seems sketchy, but people are generally friendly, just of different culture. Though it feels it may be unsafe for single women. If you like asian food, there are plenty of great options around. Alas there's a dealer shouting under the window at night, but with closed windows you can't hear him.