Marie was a very accommodating host from the moment we booked. She sent us some great suggestions on what to see and do and even made a dinner reservation for us.
The apartment is in an amazing location. It would be hard to find a more central place to stay. We walked to all of the main museums, Retiro Park, Plaza Mayor, the Palacio Royal, Puerta Del Sol, Mercado De San Miguel, the tapas spots on Cava Baja, etc from the apartment with ease.
The apartment itself is on the 5th floor of the building and there is a lift - huge plus. As some others have mentioned in their reviews, you do walk through a small portion of Marie's apartment to get into the apartment you've rented - it appears to be a 2 story unit and she resides on the floor above the unit you rent. I, personally, thought it was still very private as she has her own entrance on the 6th floor. We didn't run into her one time during our stay. The unit does have AC but we found that the building stayed cool enough with the window open and a fan to not need it. The bed was comfortable enough and there was a coffee maker. The little living room isn't much but it's fine to chill in for a bit while you plan your day.
The main takeaway is that the apartment is a great jumping off point for a few people who are wanting to see the sights in Madrid. We certainly recommend it.