The apartment is located in the heart of Madrid, having everything just some minutes walking. It was comfortable enough for the 2 of us, who stayed for 2 nights. It was clean and tidy, but would have appreciated having more room for storing clothes, at least having a drawer would be great.
The day before the check-in, we scanned our passports and have faces scanned with the app they provide you with for this purpose. This app is also the "key" you use for opening the building's front door, the apartments entrance, and the proper apartment door. If you ran out of battery in your cellphone then there's no way for you to get in :P
The major drawback it has is the noise coming from outside (since it's in a very centric part of the city with lots of bars) and from the other apartments because the walls are thing and not. As others mentioned, is not soundproofed at all.
The host, Marco, was very responsive and helpful. As a plus, they let you leave your luggage at their offices (10 minutes away walking from the apart) after you do the checkout until they close at around 6 pm.