We stood outside this building for longer than we hoped waiting for a representative from the management company to meet us. The sign outside the building that said Hostel was not auspicious and we were not encouraged by the dingy lobby or the creaking stairs to the third floor. Nor was the miscommunication about what unit we had rented. By my, once in our correct apartment it was lovely, obviously recently renovated and thoroughly modern. Large and comfortable, especially the master bedroom (king, big bathroom). The second bedroom (two twins) was small - this was probably the sole negative. It was fine for two teenagers, but not more than that. There was a nice kitchen and comfortable living room with a balcony overlooking the street. Also overlooking a popular Irish bar, which was very busy on St. Patrick's Day. But none of this street noise could be heard in the bedrooms, so that's good.
The apartment is not 50m from the Puerta del Sol and surrounded with restaurants and bars. A great neighborhood to be in and convenient to the museums and other sites. Five stars for location.
The building appears to be undergoing renovations, so perhaps in a few months it will be as nice in the common areas as in the apartment. The managers also ask for a rather ridiculous security deposit (€700 in our case), but they did refund it promptly after we checked out. All in all, a great place from which to explore Madrid.