Overall it was a great stay. Location was very central and in a great area - very easy to walk around and get to other neighbourhoods. Apartment was clean and functional, and had everything you'd need. The hosts were friendly and responsive.
The two main issues were the noise at night and the keys. The apartment is opposite a late bar so it got very loud and made it difficult to sleep at times. Also, the key to the door on the street was very difficult to use and we couldn't get back in on our first day - had to wait for someone to come help us. This wasn't explained when checking in so it caused some panic.
Keeping those in mind, it's a great option for a short stay in Madrid!