Clean and well-equipped (e.g. multiple electrical heaters, cooling systems and ceiling fans, although we needed none of them at the end of October) systems apartment. The location is also very convenient, as there are a couple of metro stations in close proximity, which will take you to the very center (think "Sol" station) in only a couple of minutes.
Also, the host Magali was very friendly and always quick to respond to any kind of request we had. Furthermore, the private parking lot was a real dealbreaker in a crowded city like Madrid.
The only downside really, was the inability to open any windows (which I personally didn't mind, but one of my family members did).
For those who are concerned about the bedding situation, the double bed is a french double bed and the sofa converts into a regular double bed (wider than the french bed) with a proper mattress.