This is a great place to stay. The apartment is compact and really well thought-out, so that it is comfortable, functional, and has everything you are likely to need. The windows are all double-glazed and block out outside noise almost completely. The building is also well built so that I never heard the other guests above and below me - and that is a rarity in places I've stayed. I was a little unnerved on arrival when I realised that there was construction going on immediately next door (I'm quite noise sensitive) however I was pleasantly surprised at how well the noise was blocked by the double glazing and it wasn't really an issue.
I was on the second floor and the stairs are very steep - I would not recommend staying here if you have a large suitcase to get up and down the stairs. With my carry-on bag it was no problem.
There is an A/C unit that also functions as a heater and it worked well. The bed is very comfortable and all the bedding was good quality. The building is just a few steps away from the Avenida metro stop and in general this is a very convenient and pleasant location. There's a pharmacy across the street, a good restaurant around the corner, and a small mini-market for basics just up the street. I would definitely stay here again if I return to Lisbon.