In general it was a lovely stay. The apartment is in a great location (short walk from train station, around the corner from the main street where you can get a taxi or walk anywhere, close to a metro station!) We were provided fresh fruit and juice/milk in the fridge when we arrived. Decorations were nice and everything was very clean. Dining table area and couch provided a nice place to relax with snacks after enjoying our time sightseeing. We were able to find everything we needed in the space (loved that there was plastic food film to wrap our left-over groceries in!)
Only MINOR downsides: 1) no space to lay a suitcase flat if it is large, but we were able to make due by opening it on the bed and unpacking into dressers and closet space. 2) Ceiling in bedrooms and bathrooms (especially upstairs) are low. People over 6 ft. tall will definitely notice as they won't have much clearance.
We stayed with 2 adults and 2 kids for 5 nights. If you are only traveling with backpacks and not planning on spending much time at home you could maybe fit another person but it would be tight.
In general, it felt like a good value for Madrid.