Overall, this is a good choice for Lisbon if you're looking for a budget-friendly, convenient accommodation to crash in at night. Some additional details are below.
The good things about the apartment:
-Location is perfect. Close to many major sights as well as 2 metro stations and a bunch of tram stops, including the handy tram that goes to Belem. There is also an excellent supermarket nearby, and also cafes, restaurants and convenience stores on the same street as the apartment (though for restaurants I recommend eating elsewhere--the ones on the same street as the apartment are tourist-traps)
-The hosts are very quick to respond and communicate well
-The check in/out is super easy, as the hosts give you a pin to access the building and apartment keys, so you don't need to feels stressed about arriving to the apartment at a particular time that's mutually convenient for both you and the host
-The apartment is very unique. It's in a super old building, with very high ceiling, and decorated very tastefully and interestingly, as you can see in the pictures (including a hammock and an elevated bed to which you climb on a circular staircase). It's actually large enough so that it could be theoretically broken into 2 or 3 separate rooms, but instead it's a studio, which gives it a very spacious and airy feel that I personally really like
-There was plenty of hot water. I like to take long, hot showers and I never ran out of hot water, not even close to it. I can tell you that it's not always the case with apartments that I stay in
-The bed was very comfortable. The mattress provides good back support and the good duvet keeps you warm and cozy at night
-Wifi worked well, I had no issues with it
Just for FYI, some things that may or may not bother you, depending on what you're looking for:
-The heat is not that great and if you're somebody who likes to be in a warm apartment in the winter you would probably find it too cold. The apartment is in an old, poorly insulated building and it doesn't have a built-in heating system (instead, there is a portable heater, which is simply inadequate for such a large place). I don't think there is anything that the host can do about it that would not involve an expensive renovation, so it's not really his fault. He's doing what he can with the portable heater and the thick big duvet.
-The kitchen area is a bit too dark if you're planning on cooking extensively and doing dishes
-The bathroom could use some updating. In particular, the floor in the shower stall is cracked and a bit rusty, and the water pressure could be better.
Like I said, it's a good choice for budget-conscious travelers. Travelers who are more picky and want to cook elaborate meals might be less happy here.